As an author on Leanpub (http://www.leanpub.com/php) and one who's book has made it onto at least one torrent tracker site, I can say that if your book is worth pirating, it will be, and by trying to protect it with DRM you are only going to lose readers. It's only my first book, but it doesn't seem to have hurt my sales figures, and I've had plenty of complimentary feedback about the leanpub experience.

Cheaters will always cheat, and let me tell ya: I've spent more money on Leanpub than I have anywhere else with "proper drm". :) I guess developers have an honor code towards their fellow software developers.

For self-published authors, obscurity rather than piracy is the largest business threat

That's even legal among close friends and within your family – at least in most European countries (private or personal use). And I consider it an absolute necessity since printed books can be easily shared and in a family, it's simply not feasible to buy the same content a few times.

I think the bigger worry should be regarding whether or not you're writing something people will buy.

The reason so many things suck these days is that people worry about edge cases, and build in rules/sucky experiences to protect against them.