Now that my Book Of Mormon phase is almost over, I'm getting ready to move on to another musical, and as a preparation, I bought Wicked.

But the blurb says this is a retelling of The Wizard of Oz, and it looks like knowing the source material would help a lot. I don't know the first thing about it.
Not. A. Thing. There's a girl named Dorothy, some sort of C3PO robot (?) and a road that is made of gold.
So, Odeck, I'm eager to learn, where do I start? I couldn't find the book, but I'm vaguely aware of an Oz movie, would that do the trick?