Here's information from Amazon about public library books on your Kindle.

A common question coming to the WPLC support email: How can I return a book early?

Some people don't understand that it's not necessary to return titles; after the checkout period expires, the file is automatically checked back in. No need to do anything. A copy of the ebook will remain on your computer and any device if you have transferred it. The copy will not open or play after the checkout period ends. You can delete the copy just like you would delete a file from your computer. That said, you can return some formats early. With the Kindle on the scene, this seems like a good time to review the rules for the different formats.

WMA (audiobooks): NO. Because of licensing agreements with publishers, WMA formats cannot be returned early
MP3 (audiobooks): NO. Cannot be returned early at this time, though there is some rumbling that this might change
PDF: YES through Adobe Digital Editions; NO from portable device

KINDLE: YES
If the recipient is finished with the loaned book and wishes to return it, they can do so from the Your Orders section of Manage Your Kindle.  Here's how:

1. Click the "+" symbol next to the loaned title.
2. Click the Delete this Title button.
3. Click Yes in the pop-over window to confirm the return.

EPUB: Yes. EPUB ebooks can be returned using OverDrive Media Console v2.3 (or newer) for Android, BlackBerry, and iPhone/iPad.
How do I return EPUB eBooks using OverDrive Media Console? from:  http://overdrive.com/software/omc/faq.aspx#question-04b

On Android…
    On the Library screen, tap and hold the eBook you want to return.
    The title menu is displayed.
    Tap 'Delete' to display the return options.
    Tap 'Return and Delete'.
    The eBook is returned and deleted.

On BlackBerry…
    Select the eBook you want to delete.
    Select Menu > Delete to display the return options.
    Select 'Return and Delete'.
    The title is returned and deleted.

On iPhone/iPad…
    On the Library screen, swipe across the eBook you want to delete.
    A 'Delete' button is displayed next to the title.
    Tap 'Delete' to display the return options.
    Tap 'Return and Delete'.
    The eBook is returned and deleted.

On Windows Phone 7…
    On the Library screen, swipe across the eBook you want to delete.
    A 'Delete' button is displayed next to the title.
    Tap 'Delete' to display the return options.
    Tap 'Return and Delete'.
    The eBook is returned and deleted.
    Tap and hold a title to show the context menu.
    Select 'delete' to display the return options if the selected title is not expired.
    Select 'return and delete'.
    The title is returned and deleted.