Prayers for Rain — Dennis Lehane

Karen Nichols was pert, blonde, in love with her fiance and her life when Patrick Kenzie first met her. But six months later, she jumped naked from Boston's Custom House, leaving behind a downward spiral of drug abuse, depression, and sexual misadventure. She was an utterly different woman--and Kenzie wants to know why.

What he finds is almost incomprehensible: a depraved stalker who carefully targeted Karen and slowly, methodically, exploited her every weakness, stripped away all that mattered to her, and then watched her self-destruct. Now as Kenzie and his former partner Angela Gennaro begin a psychological battle against a master sadist the law can't touch, they discover he's starting to learn their weaknesses, their loves -- and he's determined to tear their world apart.