What is it?
Demand Driven Acquisitions (DDA)* programs allow libraries to make eBooks discoverable to their patrons prior to purchasing. A title is triggered for purchase via significant use by patrons as defined by the library. (Source)
DDA is good for librarians and its patrons (there aren’t necessarily less costs but funds are used better); and it’s potentially good for publishers. It’s an added cost for everyone, and there are concerns from both parties (from NISO.org survey).