By Gene Wolfe, in four volumes (Nightside the Long Sun, Lake of the Long Sun, Calde of the Long Sun, Exodus from the Long Sun) or two volumes (Litany of the Long Sun and Epiphany of the Long Sun) or maybe even one SFBC volume.
I actually disagree substantially with the Inchoatus review this time. I found this to be one of Gene Wolfe's most straightforward books, but perhaps I'm too simplistic and not sufficiently interested in what the "true" story really is. I do admit that their criticism about unsympathetic characters has some weight to it, though. In the sequel, The Book of the Short Sun, the characters I found most sympathetic were the aliens. (Note: You should read The Book of the Long Sun before The Book of the Short Sun to get the most out of it.) I also found the sequel to be much more byzantine and confusing.
This is a very Christian book in some ways. Patera Silk, the priest in charge of the most impoverished parish in his city, receives an epiphany from a god known as the Outsider. The Outsider is considered to be a minor god because he is not one of Pas's children, but Silk gradually comes to believe that the Outsider is the god of all gods. References to events Silk was shown, such as "a man riding a donkey entering a foreign city while people waved large, fan-like leaves", are extremely suggestive.
Although I found this fairly straightforward, especially compared to The Book of the Short Sun, it still requires a significant amount of concentration to get through. There are times when characters act on knowledge that they don't share, and some things that just aren't explained at all. The end declares that this book is a record put together by Horn, one of Silk's students, based on his own and other witness's testimony as well as conversations with Silk himself, which casts doubt on certain parts of the narrative. This may be why Inchoatus had so much trouble with it; I don't know. There are also various details upon which light is shed only in the following Book of the Short Sun.
In addition, while I enjoyed it as an adventure, as I said, the characters themselves were somewhat lacking in sympathetic qualities.
This is definitely science fiction, and requires a substantial amount of time to read. You will probably want to have all four books on hand, as the narrative proceeds directly from each book to the next without any obvious logical division in the plot (unlike The Book of the New Sun, which was segmented at least somewhat logically, and The Book of the Short Sun, written as a sort of memoir of past events while also recording the ongoing ones in the life of the (fictional) writer, which was logically divided by where he ran out of paper.) You will probably also want to read The Book of the Short Sun (On Blue's Waters, In Green's Jungles, and Return to the Whorl) soon afterward, while your memory of the events in this book is still fresh. For that reason, I can't really make an unconditional recommendation of this; it is an awful lot of pages to commit to, although they are aguably not wasted pages, as Wolfe rarely or never adds irrelevant details. Still, I enjoyed this quite a bit and parts of The Book of the Short Sun even more.