You do not have to add bogus .txt files. If you click on the down arrow mext to the "Add Books" icon you will see there is an option to add empty book entries with no attached ebook file.
That is how I manage my own paper books, adding a "PAPER" tag so I can filter them out as required. If you later get a electronic copy of a book you can still add it via the Edit metadta dialog. I also add handling books I would like in the same way, in this case using a tag of "WISHLIST"