Falling by Christopher Pike

Kelly Feinman went rogue on her last case to hunt down the merciless Acid Man, her once-loving husband has taken their daughter and left, and now she's trying to recover from the scarring assault and find her footing by taking on a new case: the kidnapping of a baby, Jimmy Techer. Matt Connor is heartbroken when the woman he loves leaves him for another man and in his misery, he becomes vindictive and diabolical. He swears that when he's finished, Amy—his lost lover and Jimmy's mother—will know the potency of his pain.