Number One with a Bullet by Mark Yoshimoto Nemcoff

When the top ten hitmen face off in a last-man standing contest being bet on by some of the world's richest men, the only rule is to stay alive. Assassins are ruthless and clever killers who get paid a lot of money to make problems go away. And pretty soon one of them is going to be a hundred million dollars richer if they can eliminate all the competition, before the competition eliminates them. But for retired hitman Johnny Dane, considered at one point to be the best hired killer in the world, getting out of the life doesn't necessarily mean getting out of the game. Welcome to the world of high-priced hitmen and the game they've been thrust into. The last man (or woman) standing not only gets the prize but will be the only top notch assassin left available for hire. One of the top ten best killers has been notoriously absent from the contest, Johnny Dane. Having left the life, he's disappeared to an oceanfront hideaway and has vowed never to return, but when the game finds him and threatens the only woman he's ever loved, Dane realizes the only way out, is to get back in.