Check out From Zero to Xena: The Ring of Change Saga by T.F. Wright on Amazon.
This is an erotic, romantic female muscle growth transformation story. A shy, out of shape girl named Zoe has a crush on her new personal trainer. Jim, though, is tired of training people who don’t care about fitness. He longs for a protege, someone who he can mold into the perfect athlete. Jim is about to find out what happens when he gets his wish, and the magical Ring of Change begins to alter Zoe's figure and her personality. In fact, he'll wind up learning a lot more about the ring than he ever expected - he's in for a big surprise when he puts it on himself!