The Film Of The Book (usually the last one in a series) is actually two films.
So you're watching the film of the last book in a series, content in the fact that the whole movie adapts the whole book like the preceding ones. But then there is a Cliff Hanger ending. You read the source material to find that the book continues beyond that point. What happened? Well, The Film of the Book was split in half. It's becoming a common trend to see cinema adaptions of the last novel in a book series be split in two, and likely if said book is the Grand Finale (which it almost always is). Also see Gecko Ending where an adaption ends just short of the source material ending and the adaptation has to Wrap It Up. Sub-Trope of Divided for Publication.