Thanks for your question!
With the internet's incredible marketing potential, getting your book
noticed for minimal cost is easier than ever. You already have a
website, which is Step #1. Your next step, if you've not already done
so, is to index your site with search engines. Several good free
resources are addme.com, submitit.com and addpro.com.
Once your site is indexed, you'll want to boost its visibility. Here's
an article to get you started:
Get Googled and Build Credibility
Another effective approach is to draft a short press release
announcing the publication of your book, including a brief synopsis
and several testimonials. Email this to the media of your choice --
local newspapers and TV and radio stations, for example. By
positioning yourself as an expert, you can also offer your knowledge
for on-line chats. Two articles:
Creating news releases: easy and free publicity for your business
Make the Most of Your Time as an Online Chat Guest
You may also wish to join listservs relevant to writing and/or your
book, or write articles for other websites in exchange for a
byline/link to your site.
Two of the easiest ways to publicize your book are business cards and
your email signature file. Your blurb can be as simple as "So-and-so,
author of Such-and-Such. www.so-and-so.com." Even these brief nuggets
act as bait to pique the curiosity of potential customers. You can
print business cards cheaply from your home computer by purchasing
perforated business card stock at an office supply store, or buy them
cheaply from sites like VistaPrint.com. Though you're chiefly
interested in on-line marketing, distributing business cards offline
can help to spread the word. Hand them out like candy!
Here are a few more articles -- some refer to marketing a freelance
writing business, but many of the same rules apply to book marketing.
Book Marketing @ about.com
Five Lazy Ways to Market Your Business
How and Where to Get Your Book Reviewed
Internet Publicity...Where to Start
Making Valuable Contacts Online
Marketing Online is No Mystery
Maximizing the Effect of Your Freelancer's Bio
Networking is Key
Top Ten Ways to Prepare for your Online Book Marketing Success
Finally, you may wish to visit the Digital-E Biz marketing chat forum
I used the following search to find your answer:
"online book marketing"
Best of luck!