"The Informant" by Thomas Perry
"The Butcher's Boy is back. Thomas Perry's vengeful assassin, who turned on his Mafia clients and helped his author win an Edgar Award in 1983, has been quiet for a decade. Now, one of crime fiction's most memorable killers has returned to play a deadly physiological game with Elizabeth Waring, the only Justice Department official who ever believed he existed.
The Butcher's Boy knows Waring can help him hunt down the Mafia boss who sent a team of hit men to kill him - and in return he offers her key information that will help her crack an unsolved murder. So begins a new assault on organized crime and an uneasy alliance between opposite sides of the law. As the Butcher's Boy works his way ever closer to his deadly enemy in an effort to kill him first, Warning is in a desperate struggle, either to force her unlikely ally to become a protected informant or to take him out of commission for good."
Awesome book. I didn't expect the ending at all and was totally shocked by the outcome. Again not a typical book I would have chosen; however this book club has opened some new door. (smiles)