Velocity - Dean Koontz

If you don't take this note to the police I will kill a lovely blond schoolteacher somewhere in Napa County. If you do take this note to the police, I will instead kill an elderly woman. You have six hours to decide. The choice is yours. On the windscreen of his SUV, Billy finds the first note. He thinks it's a hoax. 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... Billy must think the unthinkable, fast, in an accelerating nightmare. More communications from the killer follow, with even tighter deadlines. Each is more personal, more confrontational than the last until he is isolated, with no one to rely on but himself. Finally he must risk everything to save the intended victims...