Stylistically, this book reads like a hybrid of realism and fantasy. We'll start with the realism. We get some very graphic descriptions of things, hence the detailed style, but we often don't get much detail about major events, particularly those in Elphaba's life.
This is where evasive and episodic come in to play. Evasive means that the narrative style often skirts around issues without coming out and telling us things directly. Liir's "mysterious" parentage is a good example of this. We assume and pretty much know that Elphaba is Liir's mother, but the book never definitively says this. We also get very spotty details about Elphaba's childhood, her underground activities in the Emerald City, her life as a nun, the seven years she spent learning to sew wings onto monkeys, etc.
This brings us to episodic. Episodic means that the narrative is broken up into small, stand-alone sections, or "episodes," that jump through time. We leap from Elphaba's childhood to her college years to her adulthood with some pretty large gaps in between that never get fully filled. This stylistic trait actually complements the book's fairly detached, often sarcastic tone as well as the limited narrative perspective.
Elphaba's story is told in a way that makes her somewhat enigmatic, or mysterious. We're left with unanswered questions about her, which is pretty realistic, since the people we hear her story from – Fiyero, Glinda, Boq, Nanny – don't really know her completely either.
In spite all of this mystery and realism, this book is often very funny. Take this passage with Nanny and Liir, just before Dorothy arrives:
She's sent the crows out to blind the guests coming for dinner!
She's BLINDING THE GUESTS COMING FOR DINNER!
Well, that's one way to avoid having to dust, I suppose. (5.15.15-18)
This is set right before the very tense scene in which the Witch is murdered. So humor is often used to break the tension and to give us relief from the book's more weighty elements.