"The 2nd Deadly Sin" by Lawrence Sanders

Boone was raising his head in a drunk's delayed response, features changing comically to surprise, outrage. Delaney, swinging an arm from the shoulder, hit the sergeant across the face with an open palm. It smacked the man's head around, left him quivering, face reddening. 

"Cocksucker," Delaney said, without expression.

Delaney, a retired chief of detectives in NYC, and his subordinate Sgt. Boone, an alcoholic, are partners trying to solve a high-profile killing. Boone gets shit-faced drunk. Delaney goes to his home to set him straight. He does, and then some. Delaney is a bastard when he has to be, charming most other times, but always relentless in hunting the killer. Sanders does a masterful job of character development in his 1977 novel. I don't read many 443-page books, though I'd gladly read another by Sanders. A marvelous writer who knows his stuff -- cops, crooks and often fine line between them.