I apologize for the small picture, it was the best Google Image could do.
The novel is called
Fallen Angel written by William Hjortsberg. It's kind of a hybrid of old hardboiled detective but with a slight supernatural twist and some horror thrown in for good measure. Very well written and quite unique, or at least it was for the time (It was published in 1978). It's one of those hidden gem dollar store books you buy because the cover looks intriguing. Problem is the book is out of publication, so used bookstores are the only way to go.