What does "maximum 1 book in
Library" mean?

I mean, a Library with 1 book is no library, it's just 1 book.
It would be helpful if the app description in the App Store made it clear what
exactly the differences between the free and non-free versions are. It's one of the big annoyances in the App Store when such differences are not clarified. It's not even clear to me from the post above.