I loaned out a book some years ago and now would like to read it again, except I can't remember what it was called. It was about a guy who was made to go undercover to investigate a cult. During this time he's set up on a blind date with some woman who he finds utterly boring but he is made to date her. Him and his girlfriend take lots of acid and it gets out of hand, with him seeing things regarding the other woman and the devil. His girlfriend ends up shaving her head and behaving like a dog, having to be taken outside on a lead, completely naked, to do her business, etc.