Crosstime

An orphan, Blake Walker has never really known who he was, and his strange flashes of intuition have always set him apart from those who raised him and everyone else he has known. Acting on one of those flashes, he prevented a murder. But neither the assailant nor his intended victim were from the world Walker had always known - he had stumbled onto the greatest secret of the ages.
Our Earth is only one of an infinite number of Earths, each with a slightly different history from the others, each separated from the others in a crosstime dimension. Walker was drafted into a frantic search for a madman from an advanced Earth who desires to be an absolute ruler of men. The would-be tyrant has chosen our Earth as the place where his reign will begin.
And, if the powerful technology he controls does not give him complete control of the planet, he will not hesitate to destroy it utterly.

Called “one of the most popular authors of our time” by Publishers Weekly, Andre Norton is a legend in science fiction, and was one of our greatest storytellers. She wrote science fiction novels for five decades, beginning with the now-classic novel Star Man’s Son in 1952. Many of today’s top writers, including David Weber, Eric Flint, C.J. Cherryh and Joan D. Vinge, have cited her as a primary influence on their own work. She was Guest of Honor at the 1989 World Science Fiction Convention, and has received the Grand Master award from the Science Fiction Writers of America and the Lifetime Achievement Award of the World Fantasy Convention. She was astoundingly prolific, with over thirty books in her celebrated Witch World series alone. She introduced three generations of SF readers to SF and fantasy, both through her critically acclaimed YA novels and her adult works, and remains one of the most popular authors in both fields. She died in 2005.