Another approach you can take to increase your exposure is to write articles for the various online writers sites. The rate is generally low, but that's not the point. We're not after money at this point, but the other sites' traffic back to your site. Needless to say, you should attach a comprehensive sig file to everything you write, which includes the title of your ebooks, your Web URL and email. Collect testimonials and reviews.
Word of mouth is still the best and cheapest form of advertising.
Think about it . . . Maybe the first, possibly the second and certainly the third. You can use this same principle when promoting your ebook. Once it's written, submit it to any number of web sites, ezines and online publications that review ebooks. But don't forget offline publications.