If I wanted to get really into medieval welsh literature instead of just reading everything in our beloved Hergest duo, do you have any recommendations on where to begin?
hi! sorry it took me so long to answer this but hopefully the length of the answer means it's worth the wait. by "our beloved hergest duo" i'm assuming you mean the white book of rhydderch and the red book of hergest, and more specifically the texts collected as the mabinogion from those two manuscripts - if i'm wrong let me know. i'm also assuming that you mainly want to read in english translation, at least to start with.
there is a LOT of medieval welsh literature out there beyond the mabinogion but a lot of it is harder to access. this is a rough menu of options with my honest opinions about how easy it is to get at these things:
the triads of the island of britain (trioedd ynys prydein), aka a big long list of People And Things From Welsh Tradition (Some Possibly Made Up). for this you want rachel bromwich's edition and translation: there are four different editions of this and all of them are expensive (and three of them are out of print). i recommend keeping an eye out on secondhand book websites for the 2nd edition (1978) or the 4th (2014), or bugging your library to see if they have, or will buy, either of these. if you're currently at uni you may be able to get access to an electronic version of the 4th edition.
material about merlin. maybe start with geoffrey of monmouth's latin vita merlini - this is less a reflection of welsh tradition and more an extremely lengthy riff on it, but still very interesting. a new translation of it can be found here! medieval merlin material in welsh is basically all prophetic poetry, mostly from the black book of carmarthen. at the moment, the best place to find translations of this is in the romance of merlin, ed. peter goodrich (1990) - again i recommend looking out for a secondhand copy or talking to your library. hopefully the myrddin project at cardiff will soon have fresh editions and translations for us available online! (in the meantime, here's their twitter.) there's also armes prydein vawr, a somewhat different type of prophecy poem also associated with merlin/myrddin and generally dated to the 10th century, which you can find on archive.org here.
material associated with taliesin. this comes in many shapes and sizes. first of all, there's praise poetry attributed to taliesin and addressed to the 6th-century king urien of rheged: this is mostly translated in the two clancy anthologies i'm going to cite further down, but if you want the welsh text, the best place to find it is probably in ifor williams' edition (translated into english as the poems of taliesin by j. e. caerwyn williams, available from the dublin institute for advanced studies). second of all, there's All The Other Poetry Attributed To Taliesin: for this you want marged haycock's legendary poems from the book of taliesin and prophecies from the book of taliesin. again with these i recommend the secondhand or library approach. THIRD of all, there's a relatively late folktale about taliesin (this is where ceridwen and gwion bach come in): this you can find translated in patrick k. ford's the mabinogi (which it looks like you can get as a kindle or paperback comparatively cheap).
y gododdin, the massive poetic text attributed to aneirin about A Lot Of Dead Dudes In Southern Scotland. this is a tough one to get to grips with, i'm not gonna lie. if you want to get at the welsh text, the massive modern welsh edition by ifor williams (canu aneirin) is still the best there is, but he reorders the stanzas of the poem from the manuscript pretty radically. (to see the stanzas in order, look for daniel huws' facsimile edition of the book of aneirin - or, depending on how well you read medieval welsh handwriting, check out the manuscript itself.) for translations, i recommend joseph p. clancy's, which has multiple versions floating around - there's one in the triumph tree (ed. thomas owen clancy) and a slightly less full one in medieval welsh poems (joseph clancy's big anthology, now out of print). this is the most poetic while still being largely accurate, but if you're concerned about academic levels of accuracy, then i recommend balancing clancy out with kenneth jackson's the gododdin: the oldest scottish poem, which has the advantage of being designed to be used alongside ifor williams. FOR ALL OF THESE you'll need to hit up secondhand booksellers or libraries.
early welsh englyn poetry: by this i mean poetry in englyn metre about historic figures and landscapes. as academic sources/translations, if you can get your hands on them, i recommend jenny rowland's early welsh saga poetry (1990) and patrick sims-williams' new englynion y beddau (2023), but both of these are massive and expensive. a more approachable way to get at this material may be rowland's a selection of early welsh saga poems, which is intended more for classroom use - this you can get for relatively cheap as a paperback. you might also want to check out kenneth jackson's studies in early celtic nature poetry (dated, but i think he translates some of the less-studied englyn poetry in there: again, check with secondhand booksellers) and nicolas jacobs' early welsh gnomic and nature poetry (cheaper and easier to get, but untranslated, though he gives a useful glossary so you can attempt it yourself).
additional arthurian material. this is scattered across various places and manuscripts, but some good places to learn about it, if not necessarily read it, are o. j. padel's arthur in medieval welsh literature (2013, heavily recommended, you can get it cheap as a paperback); bromwich et al's the arthur of the welsh (1991), which iirc includes patrick sims-williams' translation of my beloved arthurian poem pa gur; and the new and exciting arthur in the celtic languages, ed. ceridwen lloyd-morgan and erich poppe (2019), which is going to give you a BIG and comprehensive overview of every text arthur has ever shown up in in welsh. for the last two you definitely want to go secondhand or through a library. EDITED TO ADD: [LOUD BUZZER NOISE] I DID NOT KNOW ABOUT NERYS ANN JONES' ARTHUR IN EARLY WELSH POETRY which came out in 2019! go buy it it's a £15 paperback! an absolute steal for what you get!
high and late medieval poetry of praise, lament and love: the bread and butter of the professional poet. these can be found in various places. for the gogynfeirdd, the high medieval poets, the medieval welsh texts (+ modern welsh paraphrases) can be found in the absolutely massive series cyfres beirdd y tywysogion, but this is not something to attempt to get without a powerful library on your side. the late medieval poetry, on the other hand, is edited in cyfres beirdd yr uchelwyr and can be found online here - which was news to me! much of this material has never been translated into english. for a good selection of translations of some of the best stuff, i really recommend joseph p. clancy's medieval welsh poems (find a secondhand copy or get your library to do it for you), and/or tony conran's welsh verse. a couple of good selections of the later medieval poetry are: the poetry of dafydd ap gwilym, ALL of which is available online in translation here; loomis and johnston's medieval welsh poems: an anthology; and dafydd johnston's galar y beirdd: poets' grief, which specifically collects poets' laments for their dead children.
RELIGIOUS MATERIAL, of which there is a shit-ton. my recommendations are definitely going to be missing some stuff (e.g. soul-and-body dialogues, descriptions of purgatory, etc) but here's what i've got. for material to do with welsh saints, i recommend this website, where you can find translations of a lot of the latin prose lives of saints and quite a few welsh poems about saints as well - and if you look at the bottom you'll see it lists a few more books you might want to look into. if you want an even fuller look at welsh saints' latin lives, albeit dated, see if you can get your hands on a secondhand/library copy of wade-evans' vitae sanctorum britanniae (1944). if you like genealogies, barry lewis i believe has just put out an edition and study of bonedd y saint, the genealogies of the welsh saints, available from the dublin institute for advanced studies (though it's not the cheapest thing out there).* there is also a lot of general religious poetry, which you can find edited in marged haycock's blodeugerdd barddas o ganu crefyddol cynnar (1994) and translated in mckenna's the medieval welsh religious lyric (1991).
*i should also say that if you're interested in medieval welsh genealogies in general, you want ben guy's medieval welsh genealogy - this is very technical and probably expensive but if you really need to know who's related to who in the welsh historical imagination, it's a great resource.
(pseudo-)historical texts: there are various of these. the most famous is geoffrey of monmouth's de gestis britonum (also known as historia regum britanniae, 'history of the kings of britain') - this you can find edited and translated by reeve & wright under the latter title. if you want to know about geoffrey's work but you can't get your hands on it or don't have time to read what is honestly a massive text, then i recommend karen jankulak's book geoffrey of monmouth - super useful and you can get it cheap as a paperback. then there are medieval welsh translations of this text (all known as brut y brenhinedd), some of which go on to become chronicles in their own right (brut y tywysogion). off the top of my head there are three different versions of brut y tywysogion which you can find in a good english translation: the peniarth 20 version (edited and translated by thomas jones, edition 1941, translation 1952); the red book of hergest version (ed. and trans. thomas jones, 1955); and brenhinedd y saesson (ed. and trans. thomas jones, 1971). you might also want to check out the medieval biography of gruffudd ap cynan (king of gwynedd 1081-1137), which starts as a latin text and is later translated into welsh. the latin text is edited and translated by paul russell as vita griffini filii conani (2005); the welsh text is edited as historia gruffud vab kenan (1977) and translated as a mediaeval prince of wales: the life of gruffudd ap cynan (1990) by d. simon evans.
edited to add: [LOUD BUZZER SOUND] I FORGOT ABOUT HISTORIA BRITTONUM AND SHOULD BE PUBLICLY SHAMED. this is a ninth-century latin historical text from north wales, it's weird as hell, i love it to bits and should probably actually read more of it. currently the edition everybody uses is john morris's nennius: british history and the welsh annals (1980), which is not the most expensive thing out there but certainly not the cheapest so get it through your library if you can. this is especially useful in conjunction with geoffrey's de gestis britonum because he was absolutely using it as a source.
the hardest thing to get at on this list: translation literature. by the time we get to the red book of hergest there's been a huge boom in medieval translations of french and latin texts into welsh - and these are often really fun and interesting to read, but under-studied! this is an issue because it means i basically cannot recommend you any english translations of them. if you're still interested and you want to plough through the medieval welsh yourself, here are some texts:
cân rolant, a welsh version of the 'song of roland' aka Violence Violence Violence, edited and translated by a. c. rejhon (1984) - the only thing on this list to have a recent translation, alas;
ystorya de carolo magno, edited by stephen williams, 2nd edition (1986) - a welsh version of the charlemagne legend, this is where cân rolant comes from;
ystoryaeu seint greal, a welsh translation of two french romances, queste del saint graal and perlesvaus: you can find the whole thing in the (very old, undoubtedly outdated) selections from the hengwrt mss volume 1, y seint greal, edited and translated by robert williams (1874-6), which is on archive.org here, and the welsh text of the first part is edited as y keis by thomas jones (1992);
ystorya bown de hamtwn, a welsh version of the romance of bevis of hampton, an absolutely insane text about the worst man in the world which i love dearly: the whole thing is edited (but not translated) by morgan watkin (1958); selected bits of it are edited with a useful glossary for classroom use by erich poppe and regine reck as selections from ystorya bown o hamtwn (2009);
a welsh bestiary of love, ed. g. c. g. thomas (2008) - a translation of the french bestiaire d'amour, aka Do You Want To Hear The Worst Man In The World Tell You Dubious Animal Facts? Of Course You Do;
kedymdeithyas amlyn ac amic, edited by patricia williams (1982), a welsh version of the french tale ami et amile about two identical friends;
chwedlau odo, a collection of fables, edited by ifor williams (1958);
chwedlau seith doethon rufein, edited by henry lewis (1958) - 'stories of the seven sages of rome'.
and finally, medical texts! if you want a look at medieval welsh medical practices and you want to learn a lot of plant names in middle welsh, check out diana luft's medieval welsh medical texts, which you can find online for free here.
i hope this is helpful! enjoy Experiencing Welsh Literature and best of luck getting your hands on it!