When we briefly talked about Pratchett's early books, @cheapsweets pointed out that one of the reasons they might have grown to be disliked in recent times was due to the specific fantasy works it refered getting out of popular culture and being forgotten by mainstream audiences.
Which in turn made me look a bit at what exactly were the fantasy works that Pratchett built his Discworld out of. Because... It is not what people could think.
For example, people will go: Tolkien. And yes there are references to Tolkien's Hobbit and Lord of the Rings scattered all throughout Discworld, from Equal Rites to Unseen Academicals. By the very first duology of Discworld books you have Tolkien references. Well... Almost. See, the Middle-Earth references only really start with "The Light Fantastic", with the trolls section and the chain-smoking of the wizards, and other details of the sort. But "The Colour of Magic"? There's no "real" Tolkien reference there... Discworld did not begin out of Tolkien's Middle-Earth.
So what was Discworld made out of, by the time of The Colour of Magic? Well, in terms of pure in-text references and obvious homage, The Colour of Magic is an humoristic mash-up of...
Robert E. Howard's works, and the entire "barbarian heroic fantasy" sub-genre it spawned. Conan the Barbarian is of course the main focus, given we have two different caricatures of Conan in the duology. Kull of Atlantis might also be involved with the naming of the kingdom of Krull (which, funnily enough, is NOT a reference to the sci-fantasy movie of the same name, it is a happy coincidence). The introduction of Herrena by "The Light Fantastic" also brings to mind the "sub-genre of the sub-genre" of the "female barbarian" / "female heroic fantasy" - Dark Agnes, Red Sonja, Jirel of Joiry, and more.
When it comes to the Things of the Dungeon Dimension, and Bel-Shamharoth, people immediately go "Lovecraft"! However I would like to nuance this... I do not believe that Pratchett wanted to make a direct parody of Lovecraft's writings (which have little to do with Discworld). Sure, he did reference and parody the Lovecraft mythos as a whole with the Necronomicon and all... But it seems to me Pratchett was rather aiming at the Lovecraftian fantasy proper - aka the fantasy works set in the Lovecraftian mythos, of which Conan the Barbarian is a representative. Indeed, it is something that people tend to forget but the "Lovecraft mythos" was not ENTIRELY built by Lovecraft himself. It started out of the shared works of three authors - around Lovecraft were Robert E. Howard and Clark Ashton Smith. They participated as much to the "pop culture" Lovecratian mythos as Lovecraft himself, and their works were "traditional" fantasy unlike Lovecraft (heroic fantasy/dark fantasy). They formed a "first circle" (to which Robert Block is sometimes added), before August Derleth (self-appointed "successor" of Lovecraft) formed a "second circle" perpetuation the Lovecraftian mythos with John Ramsey Campbell, Brian Lumley or Lin Carter.
Moorcock's works - especially The Elric Saga, with Elric references and nods scattered here and there.
Jack Vance's works (especially his fantasy works, Lyoness and The Dying Earth). Outside of D&D, the early Discworld is one of the most famous depictions in culture of what is called the "Vancian magic system" (that Pratchett later abandoned), this idea taken from The Dying Earth that spells take a given amount of storage space in a human mind.
Anne McCaffrey's works - more specifically her sci-fantasy "The Dragonriders of Pern". This one is pretty easy to spot as the ENTIRETY of the Wyrmberg section of "The Colour of Magic" is a direct McCaffrey pastiche.
And, while all these works had their part in building the Discworld, there is one series that should be placed above others:
Fafhrd and the Gray Mouser by Fritz Lieber. Discworld literaly opens on a parody of Lieber's iconic duo, despite Pratchett's denegations Ankh-Morpok originally borrows from Lankhmar, and there's all sorts of references to this series throughout the duology (from the game of the gods segment to the phasing-through-an-airplane scene).
And one should be careful not to project too much: for example, despite LeGuin's Earthsea being an influence over early Discworld, it does not actually influence or take part in the Colour/Light duology. It only really starts to bear its weight by "Equal Rites" (gendered magic system, Roke/UU parallels, the Dungeon Dimension gains the feature of the Earthsea's Land of the Dead).
All of that being said, there are also fantasy works that were recognized as having an influence over Discworld but that do not get directly referenced in the parodies and pastiches. For example, L. Sprague de Camp. While he is most famous today for his works on Conan and Harold Shea, his humoristic trilogy "The Reluctant King", while quite forgotten today, is when you read it an obvious proto-Discworld and its style is very reminiscent of Pratchett's own style when it comes to narrations, anecdotes and fantasy caricatures.
And while early Discworld's style pay homage to Sprague de Camp, I do see, in terms of content, rather a follow-up to Terry Brooks. While Brooks is most famously (or unfamously) known for his "Shannara" massive series, he was one of the fore-fathers of humoristic fantasy with his "Landover" series, which the Discworld seems to have some echoes of by its early instalments.
Now, of course, when it comes to indirect references such as those one must be careful and not jump to conclusion. For example there was something "floating in the air" by the 80s resulting in a lot of similar works coming out alongside each other despite not being related (see the "Krull" coincidence). Unless we have direct evidence or word for Pratchett one must be careful. Was Pratchett aware of the Belgariad when he wrote such piece of text? Did he read "Memory, Sorrow and Thorn" before beginning such book? This is why it is important to estalish a clear and neat chronology, as well as levels of plausibility. Plus, Pratchett also was known to have checked a lot of the serial pulp publications and literary magazines in which early fantasy grew out, resulting in him picking up things from some authors that are now long-forgotten and never shined in any way... So always, carefulness should be at play here.
The L-Space website has a HUGE list of reading suggestions for fantasy works "adjacent" to Discworld, I really suggest people go check it out.
