I'm really not sure how to approach this book because, well, while the story and the intrigue are pretty good, I found it to be a real slog to get through and finish. A lot of the chapters (or even part chapters) seemed to have a short section where the characters did things and then the rest seemed to fall into some philosophical discussion on the nature of good and evil. It seemed that much of the book got really bogged down into some of these discussions which ended up putting me off the story as a whole.
It is not that I did not like the story, far from it. I thought that the concept of writing a novel of the events of [book:The Wizard of Oz] from the point of view of the Wicked Witch of the West, and in doing so giving her a name and a motive behind her actions, worked really well. It is not that this has not been done before, [book:Wide Sargasso Sea] did the same thing with [book:Jane Eyre], but what that involved giving a voice to a character (Antoinette Cosway) that spent most of her time locked away in the attic, and also by making Rochester much crueller than he appeared to be in the former book (though one still has to question what possessed him to lock her away in the attic in the first place).
However, let us consider this book rather than another book that does a similar thing: which is changing the entire nature of the original book (a concept that I learned in English I referred to a intertextuality). As suggested above, Wicked explores the concept of evil, and opens up the world of Oz and the background to the readers to help us understand why the Witch (Ephalba) was portrayed as being evil in the original. Now I am loathe to go too much into this because the musical does digress from the book substantially. For those who has seen the musical will be aware that Ephalba isn't referred to as 'the Wicked Witch of the West' (or at least not from what I can remember), and we end up sympathising with her. This is not really the case in the book <spoiler>(and, by the way, the ending is different, in that she does not climb out of a hatch in the ground and runs off with her lover, who, as it turns out, wasn't murdered by the wizard)</spoiler> because as the story progresses Ephalba becomes bitter towards her enemies to the point where she ends up taking the law into her own hands and thus alienating those who once supported her.
As I said, the concept is really good, and there is a lot of political intrigue in the story where we have the wizard usurping power in Oz, and a campaign against the talking Animals to strip them of their rights because, well, they are not human, they are animals. There is also the idea of the 'Unseen God', a not too subtle dig at Christianity, though there does not seem to be any real criticism of the religion, just an understanding of its existence.
So, in this story, which the setting is reminiscent of the late 19th Century America (and the sections of Oz representative of parts of the continental United States) explores a number of concepts, such as the dehumanising of certain races (which is a not too subtle dig at white supremacy), the nature of evil (in that the reason that the Wicked Witch is considered evil is because a group of people say that she is evil, and because they say that she is evil, people begin to believe that this is the case – though we should also note that Ephalba begins to accept this label by referring to herself as 'The Wicked Witch of the West'). The main reason that she ended up with that label is because she ended up taking a stand against the dehumanisation of the animals, which ended up alienating her from certain powerful elements in Ozian society.