"Old friends?" you say. Yes - not literally but figuratively. Let me explain. Jack Reacher is a fictional character in a series written by Lee Child. It has been a little while since I last read one of the books with him in it - that is how I became acquainted with him. So when I picked up and started reading 61 Hours, as I was reading the book I had a feeling of seeing an old friend again.
If you know anything about Jack Reacher though you would probably consider it somewhat unlikely about being an "old friend". Why? Because Jack Reacher has been a transient since he left the military and does not settle in any one place long enough to develop lasting relationships. He is pretty much a loner.
But being a reader, you sometimes develop kind of a relationship with some of the characters you read about. That is why I consider Jack Reacher as an "old friend".