Another book is The Gift of Fear by Gavin De Becker. You might not be worried about potential violence, or perhaps you are or should be, but it gives some very good advice about disengaging from the person exhibiting stalker like behavior. One review said this:
"Chapters 8 and 11 would be my second recommendation, and it goes out to all the girls who have ever found themselves picking up the phone to say "I told you not to call me!" This persistent stalking behavior is one of the predictors of violence, and how you interact with it can shape how the bad boyfriend (or even bad date) will behave towards you. Ignoring him is the best option-- it's when we continually engage, even to say "go away," that the stalker gets mixed messages and persists in his harassment. "
This is a difficult situation. Best of luck.
Grits
2010 Trek 5.2 Madone WSD, SI Diva Gel Flow
2002 Terry Classic, Terry Liberator