It is impossible for me to evaluate "The Fallen Angel" without forgetting Alan Parker's adaptation. The director of "Midnight Express", the author of the screenplay, knew how to enhance a series of narrative aspects in which Hjortsberg was rather blunt. Especially in that atmosphere that darkens and increases in the film with that displacement to New Orleans, perfectly aligned with the musical and voodoo elements of the plot.
The book is more of a detective story that is very threadbare, with a dull first person and rather gray dialogues. However, Hjortsberg managed to write powerful chapters, such as Cyphre's two speeches in front of his followers or the black mass in the New York subway. And to fill the story with references to the ending, which I don't know if they are too obvious or not, but which have made me smile more than once.
This does not prevent me from believing that with the film one gets more out of this story.