Larry Niven's series of novels beginning with Ringworld centered around, and originated the concept of a ringworld, or Niven ring. A ringworld is an artificial ring with a radius roughly equal to the radius of the Earth's orbit (1 AU). A star is present in the center and the ring spins to create g-forces, with inner walls to hold in the atmosphere. The structure is unstable, and required the author to include workarounds in subsequent novels set on it. It is a so called Megastructure.
The Ringworld is an enormous single world discovered at the far reaches of Known Space, a ring around a sun at approximately the orbit of the Earth. It is 997,000 miles wide, about 125 Earth-diameters. The total inner surface of the ring is equal to that of 30 million Earths. The ring is spun at a speed to provide 0.992G of gravity on the innerside, while 20 giant shadow squares at about the orbit of Mercury occlude the Sun to provide night. It was constructed by the Pak Protectors, now mostly extinct, who had a common origin with humans. The Ringworld is home to some 30 trillion sentient inhabitants from up to 2000 hominid species. The world is described in a series of novels by Niven, Ringworld, The Ringworld Engineers, and, after the game's publication, The Ringworld Throne and Ringworld's Children.
Ringworld Prequel series (Larry Niven and Edward M. Lerner?)
Ringworld (Larry Niven)