Starman's Quest by Robert Silverberg, Science Fiction, Adventure, Space Opera

Travelling at speeds close to that of light, spacemen lived at an accelerated pace. A nine-year trip to Alpha Centauri and back seemed to take only six weeks to men on a spaceship. When they returned, their friends and relatives had aged enormously in comparison, old customs had changed, even the language was different.

Alan was a spacer, just like his whole family -- until, suddenly and without intending to, he in turn jumped ship and remained on Earth!

There were times he regretted that. Earth was a bewildering and utterly hostile place. To stay alive, he had to play a ruthless game -- and he couldn't even find anyone to tell him the rules. . . .