I just finished Endless Things, after reading the four novels of Aegypt in succession. I've been a big Crowley fan for a while now, but I never read this cycle, which began twenty years ago (Aegypt came out in 1987)! I think you could think of the four volumes as one very, very long novel. I usually do that terrible thing where I try to compare a writer to a similar writer, but I don't think I can do that with Crowley.
To the uninitiated, this series focuses on the scholar/historian Pierce Moffett, who finds himself out of a job, then "buswrecked" in a small town where he decides to remain, or doesn't "decide" -- Moffett tends to let things happen to him , and is, in fact, almost incapable of making decisions about anything pertaining to his future plans. So, the peripatetic Moffett drifts, or is pulled, on a journey through four novels that echoes the thinking, yearning, searching quests of Renaissance philosophers John Dee and Giordano Bruno. It would take entirely too long to explain the intricacies of this story or the cast of characters, but if you are interested in such things as ancient religions and mythologies, alchemy, magic, the occult, the Renaissance, the Inquisition, werewolves, Appalachian lore, witches, angels and demons, literature, the nature of time, astrology, possession, and of course, love... well then, this might be for you. You get the picture. It's complicated, but in a delightful way.
Crowley is, by far, one of the more erudite novelists around, and you do have to pay attention. The reading is demanding in equal measure to the pleasure you will get out of it.
If my rambling is too vague, then you might want to check out the laudatory review in the Washington Post. Also look out for a piece by Michael Dirda in the Post; he mentioned it in one of his weekly online chats, which I highly recommend for book nerds everywhere. Not many mainstream reviews out there. But they're just scared of him, no doubt. Awed to silence!