Once you have read the book you have sideloaded to your eReader (or once it has expired) how do you get rid of it? No problem.
Open Adobe Digital Editions. Connect your eReader. When its name appears over on the left in Adobe Digital Editions, click on it. Now you will see everything that is on your eReader. Click on the small right-pointing arrowhead that displays when you mouse over a book. One choice on the drop-down box is "Delete". Click on that choice, and the book vanishes.