“Come off it,” said Louis Wu. “You can’t breed for luck the way you breed for shaggy eyebrows!”The archetypal "Big Dumb Object", the Ringworld of the title is a million miles wide and 600 million miles across with a star in the middle and atmosphere trapped inside by rotation. Throw in some aliens and a girl who is always lucky and you have the story. The setup is very good - Niven's universe is nicely depicted, with the two sentient alien species sharing space with humanity, and then the Ringworld itself is a truly fantastic concept. The story runs out of steam a bit in the second half, as having reached the Ringworld, our heroes don't have all that much to do except try and get off it again, and the emotional investment that readers may have made in the girl isn't really paid off. Myself, I first came to Ringworld after reading its sequel, The Ringworld Engineers, and also after having read Rendezvous with Rama, so it never made the same impression on me that it did on most people. The book was delightfully spoofed by Terry Pratchett in Strata (an early non-Discworld book) and of course to an extent in the Discworld itself, which shows the extent to which it has entered popular consciousness.
Ringworld won both the Hugo and Nebula awards for Best Novel published in 1970, awarded in 1971. This was the year that the two major awards agreed on everything, with "Slow Sculpture" and "Ill Met In Lankhmar" winning the short fiction categories. Ringworld also won the first Locus Award for Best Novel, and the Ditmar and Seium Awards. Also on both the Hugo and Nebula final ballots were Tower of Glass, by Robert Silverberg, and The Year of the Quiet Sun, by Wilson Tucker. I have not read either of them. The other two novels on the Hugo ballot were Star Light, by Hal Clement, and Tau Zero, by Poul Anderson; and the other three novels on the Nebula ballot were And Chaos Died, by Joanna Russ; Fourth Mansions, by R. A. Lafferty; and The Steel Crocodile, by D.G. Compton. I have not read any of them either. I don't think I have read any of them either. Ringworld has twice as many owners on LibraryThing, and six times as many owners on Goodreads, as all the others combined. As far as subsequent purchasing history goes, the voters got it right this time. Just in case you haven't read it already, you can get it here.
Next in this series is "The Queen of Air and Darkness", a novelette by Poul Anderson from the following year (ie published 1971 and awarded in 1972).