The two novels combined in this omnibus (Caldé of the Long Sun and Exodus from the Long Sun) comprise the second half of Gene Wolfe's long novel, The Book of the Long Sun.
I don't mind having to work to get through a book, if the reward is sufficient. The "New Sun" was not an easy read, either. However, it is not a compliment to the author to say that one must re-read his work several times to understand it. If that is the case, it simply means that the author is not writing clearly. And that is most definitely the case in "The Book of the Long Sun."
The Long Sun storyline is epic -- It's essentially about the end of a world and the struggle for a new beginning, told in the most intimate "ground level" way possible. It's about religion, power, intrigue, and even romance. The lead character, Silk, is a young man who posseses wisdom and power he does not know he has, and it's wonderful to watch him develop. One must read these books in order, as each volume of the four picks up where the previous one left off.
Wolfe creates a world (he calls it the "whorl") slowly and carefully, starting with wonderful human (and animal) characters, allowing the reader to learn by observation rather than by description. The story is intensly human, and the technology which makes this "science" fiction is revealed bit by bit. One starts by thinking the "whorl" is almost medieval, but Wolfe's misdirection here is masterful.
Truly, the Long Sun series is among the best fiction -- not just science fiction -- I have ever read. It's up there with Tolkien, Bradbury, and Asimov for sure.