To the Stars, by L. Ron Hubbard

Starts well, finishes weakly. Hubbard writes good fiction. He should have stuck to that.

Alan Corday is shanghaied and forced to join the crew of the Hound of Heaven. Alan has a fiancee waiting for him back on Earth, but because Hound of Heaven is an interstellar starship, it's on the Long Passage. The time dilation effect mean that when Alan gets back home a few months later, his girl is an elderly spinster in the throes of dementia. She doesn't recognize him.

Indeed, Earth doesn't recognize Alan, and he doesn't recognize what Earth has become. He realizes that Hound of Heaven is all he has left. He eventually becomes captain of the ship. There are various adventures, mostly dumb, and eventually (years later in his timeline, millennia later on Earth's timeline) the crew of Hound of Heaven fight a war against earthmen who try to steal their cargo.

This is not a great effort from Hubbard, but it's readable.