I've read all of Philip José Farmer's books, and of his stand alone sf novels, this is one of his best. Apparently I'm not alone in thinking this. Interzone editor David Pringle included The Unreasoning Mask in his book, Science Fiction: The 100 Best Novels, and sf author Ian Watson called it "a masterpiece, Farmer's finest."
This novel might be viewed as a thinking person's version of Star Trek's "The Doomsday Machine" or "The Immunity Syndrome"; but it's really much more than that, with its metaphysical themes and implications, as well as its well-conceived world building of alien cultures and psychological examination of human motivations.
Captain Ramstan commands a rare alaraf drive starship which allows it to jump instantaneously to distant regions of space. Just as Ramstan sets off an interstellar incident by stealing the god-idol of an alien world (called the glyfa), he is alerted that one of the alaraf ships has disappeared, a victim of a world-killer called a "bolg." What is the mysterious connection between the glyfa and the bolg, and why does Ramstan begin to have waking visions of a mystical being from his long extinguished Muslim faith? Ramstan, chased by the aliens who worship the stolen god, races across the pluriverse to find the answers.
The Unreasoning Mask is a gripping, captivatingly disturbing book. Even at his most fantastic, Farmer manages to entrance with a compelling degree of realism, in particular as regards his portrayal of human nature, which in his fiction seems to carry at least as much bad as it does good. Don't miss this darkly riveting sf adventure.