Arthur C. Clarke,
Rendezvous With Rama. A true science fiction classic from one of the veritable kings of the hard scifi genre. It involves humanity's first contact with alien life, in the form of a planet-sized, hollow cylinder hurtling through space into the sun. I just started, but it looks great so far (from Clarke, how could you expect less?).
Also, is it me, or does almost every person in this thread love fantasy? That's almost all I ever see in here.