Thinking about slowly doing a shallow dive into every song in order, though I doubt I'll make it through all the songs- to time like the present to start!
No. 001, of course is Riddles Wisely Expounded, one of several riddle songs Child collected, all part an ancient tradition in folk practices and literature, of what the academics call "wit-combat".
Child notes that ballads of this general plot are very common in German with variants also in Russian, and parallel folktales in Polish. Scots and Irish Gaelic both have similar riddle stories delivered mostly in prose but with the questions alone in verse. Some of these riddle-stories pit the protagonist against the devil or some other perilous figure in a fight for life or soul, and some are battles for love, with the winner getting the other's hand in marriage or, alternately, successfully avoiding an unwanted suitor.
Child included five variants in his initial volume:
A, B. A knight courts a woman's three daughters and marries the one that can answer his questions.
C. A devil knight visits and the youngest daughter avoids his seizure by answering all his questions and naming his evil: he flies away in a blazing flame.
D. A variant pared down to just the riddles.
E. Like C, but the sisters are posed and answer the questions together.
Child's first publishing was unclear about which branch was the oldest trunk of the tree, but he later discovered the oldest known version and wrote about it in the appendices to Book V.
It's called Inter Diabolis et Virgo (i.e. "between the devil and the maiden"), and hails from a book acquired by one Walter Pollard of Plymouth c. 1444-5 , wherein likely the lyrics were transcribed not long after:
The parties are the fiend and a maid, as in C, D, which are hereby evinced to be earlier than A, B. The “good ending” of A, B, is manifestly a modern perversion, and the reply to the last question in A, D, ‘The Devil is worse than eer woman was,’ gains greatly in point when we understand who the so-called knight really is. We observe that in the fifteenth century version, 12, the fiend threatens rather than promises that the maid shall be his: and so in E."
(the book is housed at the Bodleian library and as far as I can tell it hasn't been digitized and made publicly available beyond the first page, but thankfully Child included a full transcription in his addition.)
Type A with it's "lay the bent to the bonnie broom" rejoiner is also very notable for being the source of the tune and stylings of a prime Cruel Sister variant, but it forks off in that the courted sisters are jealous of the knight's love one so the eldest drowns the younger one, and no one does any riddling.
(this one has the devil worked back in)
Type B has a charming herbal refrain of "Jennifer Gentle and Rosemary", easily inferred to be "Juniper, Gentian, and Rosemary" and likely meant as names of the three sisters.
Also of note is the Appalachian variant Devil's Nine Questions, performed here by my artistic crushes Anna & Elizabeth:
(bonus vid of Anna performing it in a grocery store aisle while passerby customers look on in confusion here)