My local library system, Boston Public Library, has added ePub and when they did, I noticed that I now get a choice of how long I want the book for. I can choose 7 days, 14 days, or 21 days.
If you have PDF or ePub and "return" it, it will be deleted from your computer. But if you also have the book on your 505, you will find that it is still readable if there is still time left.
If you are honest, you can take out Mobipocket eBooks, strip the DRM, convert with Calibre, read and then delete.
Also, there is a limit usually of 10-14 electronic borrows at one time. I am allowed 10 at one time.
Once you have a library card, you can request eBooks. But to see what to request, go to
http://search.overdrive.com/default.asp and then find the form at the library and fill it out.