Tau Zero by Poul Anderson
Starting from a small number of premises, the story ends up taking the reader on an extraordinary journey.
A ram-scoop spaceship, capable of approaching light speed, is sent to explore another solar system. Information from a probe suggests there is a planet that might be habitable. 25 men and 25 women are sent with supplies that could allow the starting of a colony if the planet is right.
The design of the ship is such that there is separate equipment for accelerating and for decelerating. In interstellar space an accident disables the equipment needed for deceleration. As they are already traveling near light speed, the rare particles in interstellar space would make going outside to repair the damage deadly. The only way to be able to make repairs is to fly the ship to a part of the universe where there is so little matter that even at their speed it is safe to go outside the ship. But such areas are extremely distant. To reach one in a reasonable amount of time requires accelerating the ship further - so close to the speed of light that time will pass extremely slowly inside the ship compared to the stars and planets.
The people must accept a more total, irreversible and previously unplanned loss of their link to the past and home than if they had colonized the intended planet. Their only hope is to make repairs far beyond the Milky Way, decelerate wherever they can and find some distant place to settle after that.
During this incredible trip, there are issues of the impact on the emotions of those on the ship, scientific problems to be overcome, mind-boggling space and time traverse, issues of leadership, and the struggle between the will to live and despair.
In terms of space and time, it is a novel of vast scope. Its characterizations may not be as deep as in some books, but they are very effective. While such ram-scoop rockets are no longer considered so plausible for interstellar travel, the science is basically good.
At least for me, the period around which this was written (1970), was a time of many great SF works. And this is a fine example of that era.
[Perhaps I should warn you that if you know nothing about "relativistic effects", "time dilation" or the idea that the faster an astronaut travels the slower he ages, this central element in the book could make it baffling.]