I don't need that feature myself, but from the many discussions here it seems that the only viable solution, if you want to keep your books sorted and organized, is to keep them in a file management app such as dropbox or goodreader. When you want to read a book, you go to the file management app and open it in your reader app using "open in". When you're no longer reading it you can delete it from the reader app to keep it lean and clean. It will still be in the file management app if you need it again.
Not really an ideal solution, but I don't think there is a better one available right now.