WORDS
Even veteran sci-fi/fantasy readers find this book difficult. Like many books in the genre it uses unique terms and develops a vocabulary all it's own. However, in The Book of the New Sun, none of the words are made up. All of them can be found in one dictionary, encyclopedia, history book, or another. These definitions are provided to help suss out whether Gene Wolfe is simply using an odd, old word for style or because it is significant to the construction of the world and important to the story.
Critic John Clute put it really well in his book Scores:
Wolfe’s extraordinarily savvy use of obscure but existing English words to help establish that dizzying sense of distance and intimacy, antiquity and ravenous presence, by virtue of which The Book of the New Sun, like deja vu, seems as indecipherable as one’s inmost self, that which becomes, as it is more examined, more alien.