Review by Dionysos Thriambos
The monster-type of the novel is a sorceror-lycanthrope-vampire, representing the genetic recrudescence of a pre-human race that has interbred with and become submerged in humanity. These other-people are called "witches" in the story, which might largely account for Parsons' affinity for the terms "witch" and "witchcraft," despite their forceful rejection by Crowley.
The book stands in many ways as a precursor of the Anne Rice formula of "anti-horror," in which the sympathetic protagonist is a praeterhuman monster at odds with humanity. Particularly like Rice's Interview with a Vampire, the Williamson story indulges in an initiatory plot-line, in which there is a gradual induction into monsterhood. There is also a thematic and mechanical correspondence to certain initiations and epiphanies described in Lovecraft's stories (e.g. "The Shadow over Innsmouth" and "Dreams in the Witch-house"), where the narrator's horror is compounded by discovering his identity with the object of his fear.
The character of April Bell is the unequivocal scarlet initiatrix of the protagonist Will Barbee. She is the Babalon who rides him as a Beast, most conspicuously when he takes the form of a huge saber-tooth tiger and and carries her naked through the night—an image repeatedly used for illustrations in the various editions of the story. (See Sex and Rockets, pp. 59 & 210, and the current Tor edition of Darker Than You Think, pp. 135 & 143, for different versions of this "Lust Trump.") Of course the names are interesting as well: "Will" is English for Thelema, and "April" is the month of the writing of The Book of the Law. Darker Than You Think is full of an apocalyptic tone, embodied most clearly in the imminence of the reign of a witch-king called the "Child of Night." (C.f. Liber LXVI, v. 2)
All of these correspondences must be chalked up to inspiration, rather than study. Only after writing Darker Than You Think, Williamson met Parsons, and eventually attended an Agape Lodge O.T.O. function, where he was favorably impressed by lodgemaster Wilfred Smith. But he never pursued any formal studies, and was left with the impression that Crowley was best characterized as a "satanist."