Larry Niven's Ringworld - the 3rd novel in his "Known Space" series, featuring 2 characters from previous stories: the Puppeteer Nessus (from "The Soft Weapon") and Louis Wu (from "There is a Tide.") They recruit 2 more: Speaker-To-Animals (a Kzin) and Teela Brown (a human) to help in the exploration of an artifact discovered by the Puppeteers in their flight from Known Space: a ring of solid matter circling a sun - 600 million miles long, and just under a million miles wide- and inhabited. Ringworld won the 1971 Hugo award and 1970 Nebula award for "Best Novel." If you haven't read this - I strongly recommend it as a fine example of why Niven was such a "hot" author in the Seventies.