There's not much mystery as to where The Wonderful Wizard of Oz gets its title. Oz is the greatest, most powerful wizard in all the land…or at least that's what he lets people think. Thanks to the detective work of Dorothy "Snopes.com" Gale, we happen to know he's just a regular guy pretending to be a wizard. She and her friends never tell the citizens of Oz, though, so when he leaves town, his reputation remains intact and his legacy as a "wonderful wizard" is preserved.
One more small thing, in case you care. You've probably noticed that the book is best known by its abbreviated title. The longer version is the original; it was first shortened to The Wizard of Oz when it was produced as a play a few years after it was published. Guess it stuck.