From the MIT press release:
[Direct to Open] moves professional and scholarly books from a solely market-based, purchase model, where individuals and libraries buy single eBooks, to a collaborative, library-supported open access model. Instead of purchasing a title once for a single collection, libraries now have the opportunity to fund them one time for the world through participant fees. To date, over 160 libraries and consortia from across the globe have committed to support the D2O initiative,
A prominent proof of concept.