Wicked, by Gregory Maguire, 1995.
I have actually bought two copies of this book: One I bought and read about ten years ago, and one I bought recently because I loaned the first copy to someone and it never came back and I wanted to read the sequel.
Yes, this book is dark. It is also intriguing. And, quite frankly, I find the book a whole lot less terrifying than the move The Wizard of Oz. Knowing the motives may not change someone's actions, but it does go far in explaining them and even arousing sympathy for the Wicked.