To be able to associate a set of pages on a book is a great existing feature. I have been using Inkodo extensively and it is a great tool. The Import PDF PRO feature is excellent.
Overtime, I have a large number of books that makes it difficult to manage them. It would be a great feature to be able to group books under a "shelf" like concept (a sort of a super book).
For example i could group all 2017 books under a Shelf "2017 projects". Other shelf for "2017 Student reports", etc. It would provide a neat way to organize a large ammount to books on topics.