The Book of the Wonders of Urth and Sky

Wolfe does the thing almost nobody attempts: he actually writes the fictional book. The tales from the brown book are not summarised or quoted in fragments — they are set down complete, several thousand words at a stretch, as separate chapters. A reader impatient with them can skip them and lose the novel entirely, which is very much the point.

Because the tales are not digressions. They are the same story told in older clothes. Wolfe puts the myth in front of you before you have the events to attach it to, so that when the events arrive you get the disorienting sense of remembering something you have not been told yet — which is precisely the condition Severian, who forgets nothing and understands very little, lives in throughout.

Worth noticing who is doing the reading. Severian has perfect recall. For him there is no difference between having read a book and having the book. The brown book is a physical object he does not, strictly, need — and he carries it across a continent anyway, because it belonged to a woman he helped to kill.

On the readability rating. Among the highest in the catalogue. You can read several complete tales from the brown book, in full, as published. Very few fictional books give up that much.