Lee Child's website suggests reading the Jack Reacher novels in the order which Child published them, beginning with "Killing Floor." Another suggested route is beginning with "The Enemy" and "The Affair," which take place before the events of "Killing Floor."
Lee Child is the pen name of British writer Jim Grant. "Killing Floor" was his first novel and won the Anthony Award for Best First Novel in 1998. As of 2015, Child's Reacher series includes 18 novels, one film and several short stories. Child made Reacher an ex-Army MP because he needed the character to have detective skills who was accustomed to doing things in a non-civilian way.