The best classic science fiction books that bear re-re-reading

I've probably read this book fifteen times. It's world-building taken to the extreme: The ringworld is an artifact big enough to encircle its sun, with habitable area greater than a million Earths. A starship crew of two humans and two aliens set out to explore this vast habitat, encountering wonders and surprises every step of the way. The world-building is truly amazing, but what makes this book so re-readable is the character interaction. The alien Kzin approaches everything with belligerence; the alien Puppeteer approaches everything with cowardice, and the humans approach everything with curiosity and delight. 

Lord of Light came out of the 1960s, a time when the youth of the world were examining the assumptions of their parents and rebelling against "the establishment." Into that world, Zelazny introduced a society ruled by a technologically advanced elite so far above their subjects that they appeared as gods. They embraced that role, taking on the aspects of the Hindu pantheon, with Kali, Goddess of Destruction, Yama, Lord of Death, Brahma, the Creator, etc. Then there's Mahasamatman, known as Sam, Lord of Light, who leads the way to a renaissance among the downtrodden. Re-read this one for the joy of the language, which Zelazny has honed to perfection. When you open this book, you're there.

In a post-nuclear-war future filled with savagery and ignorance, a band of monks labor to preserve the relics of a forgotten age, waiting for the time to re-introduce science and literacy to a world that has forsworn both. Brother Francis discovers relics of the blessed Saint Leibowitz, the founder of the order, and that discovery sets in motion the return of civilization...and the return of the same problems that bedevil us today. Told in three parts, and with great humor, this is the story of humanity's cyclic struggle between the forces of creation and destruction. 

Theodore Sturgeon was one of the best science fiction writers of all time. When I began writing, I told myself I would consider myself a success if I could produce even one story as good as any of his. More than Human is quite possibly his best work. It's a story of the next stage in human evolution, the creation of a gestalt being whose sum is greater than its individual parts. And unlike most stories about advanced beings, these don't come to a bad end, nor do they take over the world. Sturgeon loved his characters and treated them with respect and dignity, which makes reading about them a joy. 

I can still quote the opening of this novel verbatim: "It was around the hub of the evening on the planet of Porlumma that Captain Pausert, commercial traveler from the republic of Nikkeldepain, met the first of the witches of Karres. It was just plain fate, so far as he could see." Thus opens the most delightful romp in all of science fiction. When Pausert rescues three enslaved young girls, he sets in motion a comedy of errors, conspiracy, piracy, and intrigue that expands to involve the entire galaxy. Just who are these mysterious witches of Karres, and how can Captain Pausert return them safely to their home when everyone who's anyone is out to get them...and him?