Maggie Stiefvater blogs about an experiment she devised that shows the real world effect of book piracy.

It is only about two statements that I saw go by:
1) piracy doesnít hurt publishing.
2) someone who pirates the book was never going to buy it anyway, so itís not a lost sale.
Spoiler alert: As she points out, neither of these widely held beliefs turn out to be objectively true.