Everybody Dies by Lawrence Block
A blistering Matt Scudder novel.
Matt Scudder is well and truly off the booze, but he still spends time with some of his old drinking pals including Mick Ballou - an Irish American who operates more often than not on the wrong side of the law.
Mick is worried - a garage full of bourbon has been ripped off and two of his henchman killed in cold blood. Somebody is muscling in on Micks patch and he wants Scudder to look into it. Matt reluctantly agrees to take a look but won't promise a result. On the way home he is attacked by somebody wants him off Mick's case.
The following weekend Matt's mentor from AA is shot dead at point blank range when Scudder is in the men's room of the restaurant where the 2 had met for dinner - Matt knows it should have been him. Now the case is personal and no matter that he's warned off by his ex-colleagues in the NYPD and his wife Elaine, this is one he is going to see out to the end.