I guess that is an important topic to cover as well. Let's dive in.
1. Start by writing one or two really useful blog posts on the topic. Use a provider like MailChimp (free up to 2,000 subscribers) to embed a signup form at the bottom of the post. Just something simple that mentions the product and tells them to signup to hear more about it.
2. Promote these posts on Twitter, HN, Reddit, mailing lists, etc. If it is valuable content than people will share it. I wrote an article on UX lessons from the new Facebook iOS app (http://nathanbarry.com/ux-lessons-from-facebook-ios/) that was shared over 100 times on Twitter. That article alone got me 300-400 subscribers to my book launch list.
3. Put together a more official landing page for your book. It should have information on the topics the book will cover, link to your blog posts, and have a signup link. Pick a launch date as early as possible.
4. As your email list grows continue to write blog posts to promote it. Occasionally send out useful tips, tutorials, and updates related to your book to the subscriber list. This will keep the book on their minds. The last thing you want is your book to be released and the pre-launch subscribers to have forgotten who you are. But keep the emails useful.
5. A couple weeks before launch get review copies to a 5-10 experts in your industry. If you don't know anyone, start with smaller blogs and work your way up. Give them a free copy and ask for feedback. Often this will turn into book reviews published on their site or testimonials you can use on the sales page.
6. On the day of the launch send out an announcement email to your pre-launch signup list. You should also give them a discount or special offer of some kind to reward them for signing up early (and to motivate immediate purchases). I like to have guest posts go live on related sites the same day as the launch. For my book launch on September 4th I had 5 guest posts go live that day.
That's basically all I did. To give you an idea of my influence online (hint: not much) 2 weeks before the launch my blog had 150 RSS subscribers and I had ~700 Twitter followers. Not a lot.
By launch day I had a pre-launch list of 800 subscribers + about 1,000 on my iOS Design Weekly newsletter. I was able to leverage that small subscriber base to $12,000 in sales in the first 24 hours. Even if you do a tenth of that it is still decent money. Plus my blog subscribers have doubled (still not many) and my Twitter followers have gone up to around 1,100.
Seriously, try launching a product. You will learn a ton and you won't regret it.
How did you go about marketing it?