The Tale of the Body Thief by Anne Rice
When a mysterious man begins following around the Vampire Lestat, he becomes interested in his motive. After finding out that this man has the power to switch bodies, and offers Lestat the ability to be human for a couple of days, the vampire's interest is peaked. Despite warnings from friends, he decides to go along with the man's proposition. But when it comes time to switch back, the man never shows...
