From Overdrive, December 12, 2011:

From time to time, to support the needs of our supplying publishers, updates are made to the permissions associated with content or the format of content offered. In many cases, this results in content being available in formats with wider compatibility (MP3), while in other less frequent cases, permissions become more restrictive. Recently, an update has been made to select eBooks previously offered as DRM-free to add DRM. While DRM has now been applied, expansive print and copy permissions have also been enabled to permit the broadest use of the content by your patrons.

As always, our goal is to maintain a diverse catalog, while balancing the needs of both our library and publisher partners.