my library (the new york public library) has thousands of ebooks. Most are currently in PDF format, but ePub is becoming very common (the PDFs are ok to use, the epubs are just like the purchased books from the Sony store). My wife uses her the nypl.org website to download them to her sony reader. They do not work (as far as I am aware) on the kindle, but are supposed to work in the nook as well. Its super convenient. The books expire in something like 20-21 days. The biggest snafu is that they only can lend out $num of copies at a time, but you can 'reserve' them like regular books. I'm thinking of getting the nook once it becomes widely available, because of the ease of the whole system.
Additionally, most larger library systems will allow out of area residents to purchase a membership for the use of materials. I think the NYPL charges $100 for the privilege, which may or may not be worth it depending on what one likes to read and how much.