The musical turns the world of the Wizard of Oz upside down. What was once black is now white, what was white is now black-but the world is still one of black and white. The book does not attempt to villainize Elphaba, or to make her out to be a heroic victim. Maguire's purpose was to explore the character and history of the Wicked Witch of the West in a way that fits in more closely with the original books and movie. Not to implicate her from all wrongs, but to create a scenario in which a person would be so obsessed with her sister's red shoes (silver, in the original). And, if the witch did have green skin, how would that affect her upbringing and ultimately the person she became?
The book is definitely an adult book, in contrast to the family-friendly musical. It's more dark and gritty and doesn't even feel very much like a fantasy because the fantastic world of Oz is so like our world. If I had read the book first I probably would have appreciated its approach more-it wasn't till I had time to process afterwards that I understood it better.