I disagree that that's clear. What month was it released in? What month pirated? What was the trend of sales up to that point? Even the phrase "sales immediately dropped 40% over 3 months" is unclear because things that are immediate usually don't take 3 months.
A graph showing sales with an annotation on when the piracy happened would be clear. Even better if it could be compared to similar books and sales figures.
A graph showing sales with an annotation on when the piracy happened would be clear. Even better if it could be compared to similar books and sales figures.