Here's what I remember:
- Takes place in the future on a giant, bigger-than-a-planet spaceship found and now run by humans
- Humans are genetically altered and/or are enhanced with nanotechnology and have essentially become immortal and can be healed from almost any wound
- The surface of the ship is inhabited by former humans who have mutated into a new species, and live their entire lives in a kind of space suit
- The plot involves the discovery of a small planet within the ship that the protagonist is stranded on along with many others, and they form a civilization and live there for centuries
Anyone know the name of this book? I can never remember it.