Poul Anderson is an underrated and highly flexible science fiction and fantasy writer. This is now my third book by him. All of them are good and innovative. This book is with The Forever War the best treatment of relativity that I have seen in Science Fiction.
It concerns a colony ship accelerating ever onward unable (perhaps unwilling to stop). At first time slips away slowly but eventually relativistic effects build up so that millions and billions of years slip away, the ship becomes so massive that it merely shudders when it hits a galaxy and the outside universe becomes almost unasorbable.
Anderson is from the school of Science Fiction where character can be a problem, but he is not such a bad offender as some. The book has a happy ending, which manages to be inoffensive, even though things have been getting darker and darker.
Sometimes considered a classic of science fiction, this book should always be considered a classic.