[HarleyB]

I delete/return my library books using ADE - both from the ADE library and from my Kobo when it is plugged in. If the book has not yet expired you can choose to return or delete it but if it has expired the only option is to delete.
I found out the hard way if you "delete" an unexpired book in ADE it will remain on your library listing as still borrowed until the normal expiry date. So if you are returning a book early (and want to borrow something else) make sure you "return" it not "delete" it.

To return or delete a book in ADE I display just the borrowed books (so there aren't too many!) Then hover the mouse over the book and click on the arrow in the top left corner of the book. The options are open, delete, info or return. Choose the appropriate option.

To remove the books from the Kobo using ADE plug in the Kobo and wait for it to appear in the list on the left side. Click on the Kobo so ADE displays the books on your kobo. Then click on the arrow on the top left corner of the book and delete it.

I have reborrowed the same book and had no problems. It only takes about 5 minutes for the book to reappear in the library after you return it as long as no one else has it reserved. If there is a waiting list you can go back onto the waiting list as soon as you return the book.