I've written two ebooks that have this theme, both of them about a magical artifact called "The Ring of Change."
From Zero to Xena: The Ring of Change Saga
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!
This is an erotic, romantic female muscle growth transformation story.
From Charlotte to Chavette: The Ring of Change Saga
Liam's a liar and a thief, a no good London "chav" who steals a ring that has the power to transform anyone who wears it into the perfect match for whoever gave it to them.
Unfortunately, he said his perfect woman is "twice as bad" as him! The pretty and posh lady on whose finger the Ring of Change now sits is in for a surprise. She's going to transform into a woman who is fatter, trashier, sluttier...a real "chavette." Liam should have been more careful what he wished for!