http://www.next-gen.biz/news/gdc-2011-piracy-is-not-theft-sa...
and say "there's no such thing as a lost sale", then you might not care in the slightest. But the nature of the ecosystem surrounding your book will inevitably change: as someone said elsewhere in this thread, you can't loan an eBook; a solution to that would be sending someone the PDF, but you've just turned them into a pirate.
There's no easy solution. Upselling blogposts might be a better idea; even then, it would be good to have figures.
An even more radical alternative would have been Mark Pilgrim's approach to Dive Into Python. You could give your book a Creative Commons licence and expect people to copy and remix it:
http://diveintomark.org/archives/2009/10/19/the-point
but then Pilgrim admits he's not in it for the money, so in the absence of reliable financials: who knows?
I would sell the PDF for the same price as print (or slightly cheaper) and try 2 other ways for generating publicity/traffic.
1. Serialize the book as blog post i.e. post each self-contained part as spaced blog posts, with a link to buy the whole PDF at the end of each post.
2. Publish the whole book as html, but at any given time only part of the book would be available. I would (automagically) change which part e.g. every week.
Both those tactics are meant to drive search traffic, give away some of the value of the book but not all of it, so that people have a reason to buy the whole thing, either as PDF or as paper version.