Ronald Snijder has published "Better Sharing Through Licenses? Measuring the Influence of Creative Commons Licenses on the Usage of Open Access Monographs" in the Journal of Librarianship and Scholarly Communication.
Here's an excerpt:
The application of open licenses to books does not, on its own, lead to more downloads. However, open licenses pave the way for intermediaries to offer new discovery and aggregation services. These services play an important role by amplifying the impacts of open access licensing in the case of scholarly books.