Originally Posted by dwanthny
You highlighted books in the main book list window and said remove. This removes the book, all formats, and the entry for that book.
In the future if you wish to remove all text files, highlight all books and open the edit metadata window then select the format you wish to remove from the drop down remove formats window.
Adding to this.
If you highlight all the books, then press the little arrow next to the 'remove books' icon there's a few options, remove one format(txt) or all formats except(mobi), will do what you want.