Actually, one of the things that impressed me most about the Book of the New Sun is that none of the words in it are invented, although many are extremely obscure, some use unusual lexical constructions, and at least one is an editing error.

Regardless, it's a fantastic series. I'd actually call its worldbuilding quite good as well, though I have a particular fondness for the dying Earth subgenre, and although its impact is magnified by the way ideas are introduced: setting-wise, it does perhaps the best job of implementing the old "show, don't tell" adage for writers that I've ever seen.