Multiverse

The multiverse, or "multiple (parallel) universes", is the notion that there are multiple parallel universes. It accommodates, therefore, alternate histories, the same people in different circumstances, radically different worlds, etc.

Science

• The Hidden Reality Brian Greene

SF

• DC Comics
• Diane Duane, Young Wizards series
• Barbara Hambly, Darwath series
• Robert A. Heinlein's The Number of the Beast and The Cat Who Walks Through Walls
• Diana Wynne Jones, Deep Secret
• Kay Kenyon, The Entire and the Rose series (beginning, Bright of the Sky)
• Ursula K. Le Guin, Changing Planes (a slyly humorous set of short stories on the theme)
• Michael Moorcock's Eternal Champion series, most famously, Elric of Melniboné
• Marge Piercy, Woman on the Edge of Time -- possible futures, perhaps, more than true parallel universes; unclear
• Philip Pullman's The Golden Compass trilogy
• Joanna Russ' The Female Man
• Charles Stross's Merchant Princes saga, including The Family Trade (2004, ISBN 0-7653-0929-7), The Hidden Family (2005, ISBN 0-7653-1347-2), The Clan Corporate (2006, ISBN 0-7653-0930-0), The Merchants' War (2007, ISBN 0-7653-1671-4), The Revolution Business (2009, ISBN 0-7653-1672-2), and The Trade of Queens (2010, ISBN 0-7653-1673-0).
• Roger Zelazny's Amber series