Witch World is a 1963 fantasy novel by Andre Norton. This was one of the first books I read that could legitimately be called "fantasy" that wasn't a fairy tale. I loved Andre Norton's books beginning in the 1960s. Her death in 2005 at age 93 was hard on the science fiction community.
I never read much in the Witch World series, preferring the stories she wrote that had space ships as opposed to the more magic-oriented ones.
from the back of the book:
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, however, was wild beyond any imagining -sorcery.Kirkus Reviews has an overview of Norton's young adult literature.
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 WITCH WORLD.