William Sleator

When his best friend is killed in a plane crash, Ken makes a decision: He will never die. He finds a woman who claims she can make him invulnerable to pain and death for the unbelieveably cheap price of fifty dollars. And her strange ritual works. Ken can't be burned, beaten up, or even bitten by the sharks he dives among on his spring break in the Caribbean. Ken can't die, but he can kill. As long as he's awake, he feels in complete control...