Agree with the Chandler recommendation. If you don't mind American detective fiction then the list becomes huge, but you could try Dashiel Hammett, Walter Moseley, James Lee Burke, Chester Himes or Kinky Friedman. Lot of variety out there.
Idea, with Jack Reacher it's not necessary to ready them in order, but it helps to understand what's happening in his life. Each book makes reference to events that have happened in his past and it helps to know the details of those events.
John Harvey's Resnick novels, Peter Robinson's DCI Banks, the original Morse novels, anything by Ruth Rendell, Val McDermid's Tony Hill/Carol Jordan books, historical mysteries by Paul Doherty or Susanna Gregory. But especially John Harvey.