Kelly--
The ePub, Mobi and PDF books offered by most (if not all) public libraries are part of the "Overdrive" system. So you already have access to the ones the libraries yo belong to offer. They don't have a separate "Overdrive" program for their e-books, they just use Mobipocket reader and Adobe Digital Editions.
You can check out what libraries offer Overdrive books (either audio or e-books) from their search engine at "search.overdrive.com".
Since I now belong to quite a few libraries, I use their search engine quite a bit to look for books/authors I'm interested in. If you type the title of the book or the author in, it'll come up with a list of libraries that have the book/author you're looking for (if any).
dordale