Even the Wicked by Lawrence Block
Matt Scudder is an ex-cop,ex-alcoholic,private detective-now licensed but he's still happier doing favours for friends for a flar fee than anything requiring much more in the way of paperwork.Married to Elaine,a retired call girl and enjoying connubial bliss opposite his old hotel room(now the office)he is intrigued by the problems New Yorks latest serial killer is giving his ex-colleagues.