My book is available through Amazon's Kindle.
I've participated in the Kindle Select program where you can schedule up to 5 day to offer your eBook for free downloads. The idea behind it is people will recommend it to other readers after reading it after the free promotion.
Author Marketing Club.com has free Kindle book submission tool. Essentially you'd list your free promo days on the various service links they provide and they'll get the word out. As I stated this is a word of mouth thing.
If someone has a large social network on Facebook or other websites they can post links announcing there book is for sale.
Join some author/writer LinkedIn groups and participate in discussions and always use the following as part of your signature; blah blah, author of ...etc with the book's URL underneath. This is indirect marketing.
Naturally if you have a blog, website, or even on HP you'd want to include a blurb about your book on your profile page.
Depending on the type of book you've written it may be worth it for you to sign up for free daily alerts on Radioguestlist.com. Throughout the day they'll email you small radio, blogtalk radio, and podcast hosts who are looking for guests to appear on their show. If your book is of the "self-help" or "how to" genre you may be able to pitch them a show idea which ties into the subject of your book.
The interviews are over the phone generally last from 30 minutes to an hour. The host will allow you to plug your book at the end of the show. Note: Some hosts will request a paperback book while others are fine with a PDF version to review before the show. Best wishes!