The Bookcase is block added by BiblioCraft. It allows players to display books. When right-clicked, it presents the user with an interface showing the books which are currently stored in the bookcase, as well as the inventory. The bookcase can't hold books that have not been written yet, it will however keep books from other mods as well, such as Printed Books.
There is a glitch, however, that will allow you to place unwritten books into the bookcase. If you shift-click on unwritten books in your inventory while in the bookshelf GUI, the book(s) will be placed into the bookshelf. These books will stay stacked in the bookshelf and will appear as one book when out of the GUI.