Trace of Memory by Keith Laumer

Legion was homeless and hungry and looking for a way to get to Miami when he crossed paths with Foster. Foster enlists Legion's aid in dealing with the mysterious light-beings called Hunters who he says have been chasing him, and also in recovering his lost memories. He possesses a journal with entries dating back centuries which contain the only clues to his identity. Their quest involves Stonehenge, alien space ships, far away planets and unique ways of dealing with age and life's memories. - Summary by Nick Bulka