To my mailing list, via a bunch of blog posts (on my blog), at free Webinars I've given, during a talk a gave at the hack.summit(), and on Twitter.

I tried some ads (Facebook, Google, and Reddit) for a short time, but they didn't work -- mostly, I'd guess, because the landing page is still immature, and because I didn't give away any free chapters or other elements.

The landing page is now better than it was, and people now have a choice between buying the book right away and getting a chapter's worth of exercises and solutions over a 10-day period via a drip campaign. These improvements have definitely raised the conversion levels.