Witch World (Witch World Series 1: Estcarp Cycle, #1)

Ex-colonel Simon Tregarth was a hunted man--and the hunt was beginning to come to its inevitable deadly end. Tregarth was desperate, and his situation required a desperate solution. His only alternative was wild beyond imagining--sorcery.
Simon was forced to give himself up to the mysterious Siege Perilous, the ancient stone of Power. It would judge him, determine his worth, and then deliver him into a world in which his mind and spirit should be at home.
Simon Tregarth's lot would pit him against an uncanny world where the laws of nature operated... differently. Where in fact, "magic" was science.
For Simon Tregarth there would be no return, he could never escape from the WITCH WORLD.