Rendezvous With Rama Reader

In the year 2131, a fast-moving object is detected entering the solar system. Asteroid 31/439, renamed 'Rama', is set to enter the Earth's orbit within the year. A hastily arranged space flight is organised to intercept it. The scientists on board soon discover that Rama is not a dead lump of rock but a huge hollow cylinder spinning along a planned trajectory. Who or what built Rama and why?