I'll throw in the Rama trilogy by Arthur C. Clarke and Gentry Lee: Rama II, Garden of Rama, and Rama Revealed.
Clarke's original Rendezvous with Rama can be a tough read (I recommend skipping it actually), but working with Lee on the trilogy paid off. It's far more readable, but still maintains the ambitious scope typical of Clarke's work. The end of the last book tackles some seriously epic questions about the universe, and the last line is one of my favorites:
"And understanding is happiness."