Steel Magic is also known as Gray magic.
A final stand for Avalon...
Sara, Greg, and Eric Lowry are exploring the woods near their uncle's Hudson Valley estate when they are magically transported to the land of Avalon. There they meet Huon, Warden of the West. When he tells them that the forces of darkness have stolen the three talismans that protect Avalon - King Arthur's sword, Excalibur; Merlin's ring; and Huon's horn - the children set off on a quest to find the three tokens of power. For Avalon stands as a wall between the Dark and the mortal world. And if Avalon falls, so does Earth.
Andre Alice Norton (1912-2005) was an American science fiction and fantasy author (with some works of historical fiction and contemporary fiction) under the noms de plume Andre Norton, Andrew North and Allen Weston. Norton published her first novel in 1934, and was the first woman to receive the Gandalf Grand Master Award from the World Science Fiction Society in 1977, and won the Damon Knight Memorial Grand Master Award from the Science Fiction and Fantasy Writers of America (SFWA) association in 1983.
Andre Norton. Wikipedia.