It's hard to label
The Gutter and the Grave by Ed McBain forgotten since it was re-released by Hard Case Crime in 2005. I would make the case, however, that McBain's Matt Cordell should be in the discussion for the greatest PI put to paper.
The Gutter and the Grave was firsted published under the title
I'm Canon - For Hire in 1958 by the great Ed McBain. Matt Cordell (in the pre-Hard Case edition known as Curt Cannon) is a successful private investigator in New York City until one day he comes home to find his wife in bed with one of his operatives. After pistol whipping the man, he loses his license and his wife. His life spirals out of control and finds him disgraced and alone on the streets. That is until Johnny Bridges, an old friend from the neighborhood, finds him on a park bench, drinking another day away. Initially asked to help Johnny find out if his business partner is stealing from him, Cordell is sucked in to a murder mystery when the partner is found dead, with Johnny's initials scrawled in blood next to him. From there Cordell reluctantly takes the case and the story takes twists and turns, with the truth a hard thing to come by.
The premise is nothing too original but Cordell as a character is the real highlight. As with any hardboiled detective worth his salt, he is tough as nails but soft as butter when it comes to woman. Through out the book Cordell has numerous run ins with woman, with some looking to change him and others looking to use him. He also had a brutal side, as evident by the pistol whipping, that seemed to be mostly uncommon for the PI's that preceded him.
I'd be interested to see if Cordell had some influence on Lawrence Block's Matthew Scudder. Both are brilliant detectives but horrible events in their lives have turned off their will to care. If, as Raymond Chandler said in
The Simple Art of Murder, "Dashiell Hammett gave murder back to the people that do it for a reason, not just to provide a corpse", McBain gave the hard stuff back to PI's that do it for reasons other than just quenching their thirst.
The Gutter and The Grave is the only full length novel featuring Matt Cordell, with six short stories collected in 1958's
I Like'em Tough. It's a real shame considering, in my opinion, Cordell could have been one of the greats of the genre had he been fully fleshed out.