From the Overdrive blog:
On Friday, Boston Public Library became the first OverDrive library partner to add more than 15,000 public domain eBooks from Project Gutenberg to its ‘Virtual Branch’ website—at no cost to the library. This featured collection, currently in beta, enables users to discover and download thousands of DRM-free EPUB eBooks without holds, waitlists, or authentication.
That's great, but I'm looking forward to when Overdrive add other sources. Assuming Overdrive did it right, it shouldn't take any more effort to add, for example, Feedbooks.
Let me tell you why it's easy. Have you ever used Aldiko or Stanza? Did you notice how you can access the Feedbooks, Fictionwise, Gutenberg, or a bunch of other sites form inside the app? You can that because the app and the sites all follow the OPDS standard.
Hopefully Overdrive also followed the standard.