Here's one of these studies (a report from the European Commission):
http://ftp.jrc.es/EURdoc/JRC79605.pdf
The core points are that DRM discourages sales, piracy increases sales, and that the financial losses due to piracy are not as significant as claimed by the industry. However, it is in regard to music piracy, not to e-books. While there are many parallels (more than between software and music), there are some differences, too (e.g.. you won't "hear" an e-book and then buy it -- most e-book stores do offer samples already, and we don't read the same books over and over).