Ringworld series

"Louis Wu, accompanied by a young woman with genes for luck, and a captured kzin - a warlike species resembling 8-foot-tall cats -- are taken on a space ship run by a brilliant 2-headed alien called Nessus. Their destination is the Ringworld, an artificially constructed ring with high walls that hold 3 million times the area of Earth. Its origins are shrouded in mystery."--Amazon.com.