I have found, in the books of his that I've read, that Gregory Maguire's writing style is a bit... hmm, heavy and slow. That's not to say I don't enjoy his work, and in fact I quite liked Mirror, Mirror. It does take a little more patience than some more accessible stories, so if you haven't read any of his stuff, be prepared for that if you pick something up.
Having said that, he does spin a good story.
This is both the tale of the fair and innocent Bianca de Nevada, and the woman who becomes her guardian, Lucrezia Borgia. Not precisely a stepmother, but something of a mother figure, Borgia is an actual historical figure, whom Maguire has bent a little for the sake of his story. However, the Borgia family was known for poisoning their rivals and being absolutely cutthroat to get what they wanted--so it's not so much a stretch to imagine her as the villain in a Snow White variant.
Maguire's fascination with her comes through, and the story focuses on Lucrezia almost more than Bianca. She comes across as both ruthless and sympathic; it is an excellent character study.