We report the results of an experimental study analyzing the effects of Internet piracy on book sales. We conducted a year-long controlled large-scale field experiment with pre-treatment pair matching. Half of the book titles received experimental treatment, in which a specialized agency would immediately remove any unauthorized copy appearing on the Internet. For the other half we merely registered such occurrences, but no countermeasures were taken. For all the titles we obtained print and e-book sales statistics from the publishers. We find that removal of unauthorized copies was an effective method of curbing piracy, but this had no bearing on legal sales.