Why did I love this book?
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.