Velocity

William Wiles is an easygoing thirty-something, a bartender who lives a quiet life alone until a serial killer singles him out -- not to kill him, but to force him to decide who the next victim will be. On his SUV Billy finds the first note:
'If you don't take this note to the police and get them involved, I will kill a lovely blonde schoolteacher. If you do take this note to the police, I will instead kill an elderly woman active in charity work. You have four hours to decide. The choice is yours.'

Billy pays an informal visit to an acquaintance, Lanny Olson, who is a policeman, and who thinks the note is a prank. The schoolteacher dies. The next note reverses the choices: if Billy takes the note to the police, a mother of two young children will die. If he doesn't, an unmarried man who won't be much missed will die. Lanny has to take this note seriously but the deadline runs out before he can decide how to make his involvement official. Billy doesn't hear from him again because Lanny himself, unmarried, who will not be much missed, has become the next victim.

There will be more communications from the killer, more hideous choices, with ever tighter decision times, and with each choice Billy is drawn deeper into an accelerating nightmare, which steadily becomes more personal, more confrontational, until he is isolated, with no one to turn to and no one to rely on but himself. Finally he must risk everything to save the intended victims ...