The Left Hand Of Darkness is today's science fiction masterpiece, by Ursula K. LeGuin.
While her books have spaceships and stuff, her real alternative worlds are more about alternate social makeups. In this book, Genly Ai, a male human, lands on a planet of adrogynous people who only show maleness or femaleness during "kemmer", each individual's mating time.
Thanks again to the Saint Louis Public Library. Support your local library!