Amergin,

This sounds like Andre Norton. The series, I think, was the "Magic" series, including Fur Magic, Octagon Magic, Lavender Magic, Steel magic, and some other titles. Didn't "Steel Magic" have a quest for Merlin's Ring in it, along with a horn and excalibur? And there was a separate Andre Norton book called "Merlin's Mirror."

There was also a book by H. Warner Munn called "Merlin's Ring" and another called "Merlin's Godson," but I'm not sure if I ever read them...

any of these ring a bell?