Can someone suggest a crime fiction novel, with a hard boiled private investigator/detective in an urban criminal underworld.

After reading Murder on the Orient Express by Agatha Christie, I am interested in a novel with a similar detective character, but not necessarily a ‘whodunnit’ – although I am not opposed to it. More specifically, I am picturing something like a private investigator, navigating an urban criminal underworld.
Thanks

by Tyrannosaur08