Anecdotally, one of the ISVs over on Programmers.stackexchange had a piracy problem; when he started detecting pirated software and reminding the user to buy, not steal, his sales numbers directly went up.
I think this idea that downloading IP for free is ok is really wrong. If you want something, you should pay the price for it, whether the price be fixed or negotiated. IP piracy is theft.
Personally contacting potential buyers might incite them to buy your book, no matter if they were pirates before or not. It doesn't necessarily scale to do that. Though some authors actually do promote their books (go on tours and read from the books, for example).
I don't want to defend downloading, but it is a fact of modern life, so complaining about it is unlikely to help.
If 800 people a week were downloading something of mine, after I was done dancing in the streets with joy, I'd try to figure out how to convert those downloads to fans, then sales.
http://en.wikipedia.org/wiki/Public_Lending_Right
And a single copy can only service a small number too, not an infinite number, as only one person can be in possession of it at one time and usually borrow it for weeks at a time.