É Marrow por Robert Reed . Bom livro.
When a jovian sized, artificially-created structure enters the galaxy, a society of technologically advanced humans (capable of interstellar flight and functionally immortal) are the first to intercept and investigate it. Finding it to be an intergalactic ship, they decide to convert it into a cruise ship, inviting alien races to join them in its massive, uncharted interior as it makes a slow circumnavigation of the Milky Way .
After thousands of years, with over 200 billion creatures living in its upper levels, a group of explorers discover a planet hidden in the core of the Great Ship. As they explore it, however, an ionic blast cuts them off from the rest of the ship and destroys much of their technology. Because this planet, Marrow, is slowly expanding, the explorers reason that a new bridge can be built in another 5,000 years. They thus begin a civilization on the surface of Marrow.
Há uma sequência, O Bem das Estrelas , e também um livreto contendo uma novela no mesmo universo, Mero .