What is the name of this book about astral travel?

In the book the husband's wife is heavy into astral travel.  She has many books around the house and he picks one up and starts to read it.

Eventually he learns how to travel himself.  He starts out buy just leaving his body.  Then he ventures farther and farther.  He has to be careful because there is an evil entity that consumes people who becomes lost on the astral plane.

One day he travels a long distance and when he returns to get back in his body he can't.  The wife had tricked him into getting involved with astral travel and when he was out of his body she allowed another traveler to enter it.

The book ends with the wife and her new husband having sex while the old husband flees from the evil entity.