If you haven't seen it, there's a great companion book "Lexicon Urthus" [1] that provides a definition for all of the unusual English words used, as well as character references and in-book lore. I re-read BOTNS with the Lexicon, and really expanded my appreciation of the use of language in the books.

[1] https://www.amazon.com/Lexicon-Urthus-Dictionary-Urth-Cycle/...