I didn't know before seeing this movie that A Walk Among the Tombstones is actually a book - #10 in the Matt Scudder series, by Lawrence Block. Interestingly enough, the film works fine as a standalone movie, with Liam Neeson (a go-to action hero, if ever there was one) in the part of Matt Scudder.
Matt Scudder (Neeson) is a retired NYC cop who goes to Alcoholics Anonymous. He was asked to leave the force eight years ago, when he gunned down two criminals as they fled the scene; one of the bullets took a "bad turn" and killed a little girl. Drug trafficker Kenny Kristo (Dan Stevens, also starring in The Guest this weekend) asks him to find the murderers who killed his wife - he paid them $400,000 to get her back, and instead they left her body parts for him to find. Matt is able to track down one of the perpetrators, but he was only the driver, and that lead soon runs out; when the murderers abduct another girl, however, he intervenes to try and get her back safely to her family.