It's become a trend for book adaptations, especially the last in a series, to be split into multiple movies: Harry Potter and the Deathly Hallows parts 1 and 2, The Hunger Games: Mockingjay parts 1 and 2, the three The Hobbit movies, etc.
What was the first film adaptation of a book to be split into multiple movies?
- It must be a single book adapted into at least two movies.
- It needs to recognizably follow the complete plot of the book. (For instance, I wouldn't count Starship Troopers 1, 2, and 3 because they don't really have anything to do with the book).
- Multiple stories in an anthology having separate adaptations don't count, although one story in an anthology being broken into multiple movies would.
- The films don't have to have been filmed back to back, but subsequent films should be telling more of the story from the same book and intended to be in continuity with each other.
- When in doubt, the spirit of the question is something along the lines of the movies mentioned earlier in this question.