Rendezvous with Rama by Arthur.C.Clarke

Far out in the solar system and travelling fast, a large object is detected which will pass near earth. An expedition to meet it discovers that it is giant spaceship with an artificial world inside it, complete with it's own gravity. The space craft seemingly expects them, but does not appear to have aliens on board - or perhaps they are just hiding. With no way to control the ship it will soon pass out of the solar system, and team race to unlock as many of it's secrets as quickly as they can.
The expedition team has plenty of surprises as they try to gain an insight to the alien shipbuilders, including a nice touch when they are reluctantly forced to abandon ship. As it's final bow to Solar System, the craft turns towards the Sun for re fuelling.