Blood of Amber - second book of the "Second Chronicles of Amber"]: here's part of the flap-copy from the first hardcover, for a summary/set-up:

Forced out of the comfortable mold of his life in our world, Merle, son of the great Corwin of Amber, is harried across the worlds of shadow by unknown security forces, ending in his imprisonment in a cave on a mysterious shadow world [at the end of Trumps of Doom.]

As Blood of Amber opens, Merle escapes from the trap to return to Amber, the wondrous center of all realities. But Amber has become for Merle a world of dangers and ambiguities, where friends and foes are sometimes indistinguishable. A man is out to kill Merle - is it his best friend? A woman is out to help Merle - but she can inhabit the body of any woman and there is no way to tell who she is at any time.

The world of Amber is one of the most sophisticated and popular world-building concepts to engage the interest of readers in the SF world for the past fifteen years. It is a world in which magic works but technology does not. It is ruled by different physical laws and governed by one powerful family of warring princes, who alone are able to move through an almost infinite number of "shadow worlds (of which our own is one) through their own genetic abilities and magical training. Amber is an inexhaustibly rich setting for fantastic adventures.