Rendezvous with Rama by Arthur C. Clarke

The setting is that humans have settled Mercury, Luna, Mars, some asteroids, and some Jovian and Saturnian moons. Each group sends an Ambassador to the United Planets, which meets on Earth's moon. Suddenly, an enormous cylindrical space ship enters the solar system and appears completely passive; it is named Rama as that is the next name in the nomenclature sequence.
Commander Norton commands the only space ship capable of docking with Rama, so he is ordered to investigate. What he finds is amazing, yet puzzling. He encounters an alien technology that is baffling, yet they find it fairly easy to understand some of what it does. The crewmembers explore Rama, finding their greatest problem to be coping with the sheer size of the vessel.

An easy to read, simple story with hidden depths. I finshed this book during my break at work and was sorry I hadn't brought the next in the series with me.