NISO has released Demand-Driven Acquisition (DDA) of Monographs.
Here's an excerpt from the announcement:
This is a new project to develop recommended practices for the Demand-Driven Acquisition (DDA) of Monographs. Many libraries have embraced DDA (also referred to as patron-driven acquisition) to present many more titles to their patrons for potential use and purchase than would ever be feasible under the traditional purchase model. If implemented correctly, DDA can make it possible to purchase only what is needed, allowing libraries to spend the same amount of money as they previously spent on monographs, but with a higher rate of use. However, this model requires libraries to develop and implement new procedures for adding titles to a "consideration pool", for keeping unowned titles available for purchase for some future period, often years after publication, for providing discovery methods of titles in the pool, establishing rules on when a title gets purchased or only temporarily leased, and how potential titles are discovered, and for handling of multiple formats of a title.