Currently the only way to organize books is by the “bookshelves”. Books can only be added there from context menus of individual books in the main library view. But if there are many books, this becomes impossible: you’d have to scroll through many pages to even find the book to add to a bookshelf.
Minimum necessary improvement: enable for the search results view the same book context menu that works from the main library. I.e. the users should be able to search, long-press on a book and add it to a bookshelf.
Ideally: read the shelves content in from user-editable setting files that could be processed on the desktop computer, so that we wouldn’t have to do this manually. Say, a simple “shelves.yaml” file, formatted like this:
"bookshelf1": ["/Books/book1.pdf", "/Books/book2.pdf"]
"bookshelf2": ["/Books/book2.pdf", "/Books/book3.pdf"]
Note that these are not real folders, so no reason why one book must be limited to only one shelf. This would provide a much needed tagging functionality.