This recent study is relevant:
The Effect of Piracy Protection in Book Publishing
The study won't convince anybody here concerning DRM because the piracy protection mechanism it studies is issuance of takedown notices.
Also, there were too few titles in the experiment to say much about subgroups. You can interpret it to say that piracy harms bestseller sales while benefiting authors with low sales. However, the evidence that it benefits low sales authors is of questionable statistical significance.
The study does prove that book takedown notices reduce piracy and, overall, increase book sales.