Velocity - Dean Koontz

Velocity is an astonishing, fast-moving story of an innocent man forced by a serial killer to choose who will be murdered next.

On the windscreen of 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 somewhere in Napa County. If you do take this note to the police, I will instead kill an elderly woman active in charity work. You have six hours to decide. The choice is yours."

Billy takes the note to an acquaintance, Lanny Olson, who is a policeman - he thinks the note is a prank. The schoolteacher dies. The next note reverses the scenario. 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 Olson has to take this note seriously but the deadline runs out before he can decide what to do. Billy doesn't hear from him again because Lanny himself, unmarried, who will not be much missed, has become the next victim.

More communications from the killer follow, 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...