By Michael Moorcock. You can't talk about multiverses and not mention Moorcock. This is a sprawling series that has heroes jumping all over the universes and time itself. While many of the novels are more fantasy, there are a few that have some sci fi elements.

By Kathleen Goonan. Part historical fiction and part sci fi tale with a bit of multiverse thrown in—though the characters don't quite understand that they are experiencing the multiverse.

By Ian McDonald. There are three different story lines and sets of characters that will eventually converge in interesting ways. Spoiler, the multiverse comes into play in a big way with an Amazon frog.

By Philip K. Dick. A central theme in Dick's work is that there is no single base reality, but rather a multiplicity of realities that are all equally real. The novel is about a world where the Nazis and Japanese won WWII and the alternate reality where they didn't.

By Justina Robson. The first book in the Quantum Gravity series, begins after a quantum bomb tears about the fabric that keeps different universes/dimensions apart. A scientific premise with quite a bit of fantasy elements included.

By Timothy Zahn. This series of stories feature a faster than light drive with an interesting side effect—when activated you see alternate versions of yourself. Travel to these alternate universes is not easy, but you can peer into them.

By Frederik Pohl A fun take on the multiverse theme—a great scientist invents a machine to travel between universes, only to have it stolen by another version of herself who wishes to use it to become a multiversal conqueror.

By Philip Pullman. Characters travel to many alternate universes in this young adult series with the use of a few powerful items.

By Robert A. Heinlein. This book manages to unite the timeline of every story Heinlein ever wrote. It follows four geniuses who embark on a crazy romp through the multiverse and discover that every fictional universe ever imagined is real.

By Michael P. Kube-McDowell. The gate house is a guarded secret because of its militaristic potential. But for one member of the Guard, the gates are a chance to start his life over.

By Roger Zelazny. Science Fantasy about the multiverse and the one world that spawns them all. A classic and highly recommended for any science fiction or fantasy fan.

By Stephen King. The multiverse, parrellel universes, and a gun slinger that can save the multiverse. What more can you ask for? The Dark Tower is the single body of work that ties together every book ever created by King. Read it.