In Meribook, the main concept revolves around the organizational structure found in 'books' which is based on chapters and sections, but this level of organization is not limited to just books and hence the Book Type option.
Courses also have similar structure, where 'chapters' can be referred to as 'modules' and 'sections' as 'units' or 'lesson'.
The important thing to realize is that this structure has not changed, just the terminology and the context.
This is why we have 'Book Type' to separate out Books, Courses and Other resources.
This is especially useful when you make the book Public so that we can display them in the appropriate section for discovery.