Wizard of Oz.

Like many people, I was familiar with the film, but had never read any of the books in the OZ series, so I gave this one a shot. I was pleasantly surprised to discover hos different is was. The plot's essentially the same, but expanded on, and we get a lot more world-building. Dorothy, also, comes across as a much stronger, more realized character, and her increased self-assurance really makes for a more enjoyable story.