The majority of the book, as far as I can tell, is entirely original, which makes it seem less like professional fanfic and more like a story based on a model. Especially because Maguire clearly imposes a lot of his own ideas onto the preexisting construct, whether or not it's a natural fit. He reimagines Oz as a country with real-world political issues. Elphaba—which I only learned at the end of the book has its stress on the first syllable, not the second—becomes a champion of Animal rights (in Maguire's Oz, Animals are distinct from animals, which aren't self-aware and don't talk). The "Wonderful" Wizard of Oz is essentially a despot. There is talk of secession. There's rebellion and assassination and spying and all that good stuff.
It's much headier than you'd expect. Besides the political themes, the other major component is a continuing discussion of the nature of good and evil, which is appropriate when telling the story from the perspective of the villain of the tale. How and why does Elphaba turn into the cackling monster with an army of flying monkeys? Is she truly wicked or just misunderstood? A lot of the dialogue on this subject seems forced, as if Maguire is trying to stick sections from an essay of his into the mouths of his characters, but the discussion is still rather fascinating.
Maguire also injects an air of ambiguous predestination into the story, with lots of mystical occurrences and recurrences. The mysterious Clock of the Time Dragon plays an important role. And there are implied connections between various characters and events that even freak out Elphaba herself. What's kind of awesome is that Maguire never comes out and confirms anything; I think not giving a definite answer about what is or isn't going on really works for this story. It fits the style and tone very well.
There is, of course, some non-ambiguous, extratextual predestination. We know Elphaba and Glinda become enemies, even though they start out as college roommates. We know Elphaba's sister, Nessarose, gets a house dropped on her. We know she's wearing some pretty fine slippers that Dorothy ends up with—and let me tell you, I have never read a book so concerned with fucking shoes before. We know the Wizard's not all he's cracked up to be. And we know Dorothy kills the Wicked Witch of the West. This, of course, is the biggest spoiler you come in with, and it hangs over the entire book. Because you really start to care about Elphaba! She's kind of cool! She's headstrong and snarky and anti-establishment and, okay, she's also kind of annoying sometimes. But she's a very well developed character, like many others in the book. Maguire fashions people out of archetypes.
I found the structure of the book interesting and maddening. The book is divided into five sections, but between each section, Maguire skips many years. We don't actually get to see Elphaba's childhood; we skip from baby Elphaba to college-bound Elphaba. We hear a little about her childhood from other characters, but I suppose it's not all that important. More important is the huge change in political climate, which, again, we don't get to see happen. Every new section, we have to glean what major events have happened in the interim. I guess the narrative doesn't suffer since the story still gets told, but I can't help but feel I missed out on stuff.
I really liked the first half of the book, but the second half is much slower and more politically oriented. And things get a little...kookier. That is, most of the book is a solid fantasy novel, but he introduces some new elements in the latter half that seem a little incongruous. I did say I liked the mystical hoo-hah, but it still feels somewhat off at times.
All in all, I think Wicked is a book I admired more than I enjoyed. Maguire spins an impressively complex yarn, working off a framework but putting a very original spin on it. And the prose displays a love of the story, the characters, and the world he's created. The strong political focus is just not my thing, however, as well done as it is. Luckily, Maguire is a great storyteller, and he knows how to pull you in anyway. He takes you through a whirlwind journey from the birth to the death of the Wicked Witch of the West. And I've got to give him props for knowing the perfect way to end the book.