Eridan Ozma: Prisoner on Xiotan, Book One paperback version

Space opera. Dystopian sociology study. Alice Roanoke, a girl of 25 from Ohio, is abducted by a UFO and taken to the planet Xiotan. This is in 1978. She is held in a high rise prison by unseen beings. This takes place in a gigantic urban sprawl. Soon, she must learn how to survive in an alien culture millions of years ahead of our own. She meets their leader, Empress Zenobia. Soon, an even stranger Race from another star system invades this world. Civil war breaks out. Who takes the Empire?