If I must pick one without knowing why, I would likely pick Ark by Stephen Baxter. The story is hard sci-fi and is, very broadly, about a multigenerational spa

If I must pick one without knowing why, I would likely pick Ark by Stephen Baxter. The story is hard sci-fi and is, very broadly, about a multigenerational spaceship which travels across the stars following a planet-scale catastrophe. The writing and the world building are amazing and i've already read it probably a dozen times. I vaguely suspect if I can only have one book forever, some shit has gone down and I'd want a book like this. Ark is actually the 2nd book in an unfinished trilogy, Flood precedes it and is a really great read. Baxter's NASA Trilogy (Voyage/Titan/Moonseed) also deals with similarly isolated individuals and is a great read, though the ending is... trippy. The Sky So Big and Black is also a book I can infinitely re-read. There's a pervasive sadness to the book which I enjoy. It's the fourth book in a quadriligoy but the first four are neither required nor are they as good as the final installment. I'll also throw out Alas, Babylon by Pat Frank. It's a great read and depicts another scenario where shit has gone down and perhaps you're left with one book. The book talks about the tragedy of what would happen after the fall of civilization but also talks about (despite loss) people could be happier with a slower simpler life, which is deeply ironic considering it was published in 1959.