Strong reasons to use Books whenever controlled branching is not required:
For authors: Maintainability. It is truly hard for most people to edit a lengthy Lesson and understand the path through different types of pages. The entire Lesson is displayed on screen after every page-edit.
For learners: Immersion. Much more of the screen is wasted (unavailable for content) in a Lesson, than in a Book chapter. Some HTML layout does not work as well in Lesson as in Book.