I read this book because it was amongst the interesting-sounding novels discussed in A Short History of Fantasy — of course, in certain respects this sounds more like science fiction, set as it is in the extremely distant future. Nonetheless, the feel of the novel is that of fantasy, with a certain amount of low-tech material culture.
As the title of the quadrilogy says, this is the the Book of the New Sun. Some of the fantastic elements derive from the transplantation of extraterrestrial species on Urth in the generations of human space travel. Other things are possibly due to evolution. The inorganic elements of fantasy are a similar combination — extraterrestrial artefacts and creations of the deep history of human tech.
But none of this is why I heartily recommend this book. All if it is, however, partly why this is not your run-of-the-mill piece of SF. Nevertheless, the unfolding of Severian’s narrative from the Citadel and beyond is itself compelling. Once it gets going, the plot hooks you and pulls you along with it; the world of Urth and its dying sun is revealed and unfolded before you as you learn more about the main character and the various other people with whom he interacts in the Citadel and city of Nessus.
I admit, though, that I found it awkward for the first few chapters. Part of the feeling of the exotic that Wolfe gives us is the use of Greek-based words for stuff that does not exist in our world, and not always with a description of this hitherto unknown plant/object/animal. This, and the whole in medias res made the very beginning a bit difficult. But once the scene is set, and the world unfolding, then the plot gets moving and hooks you, pulls you in, and you follow Severian on his journey…