Begin marketing your new book online by creating a website with a shopping cart. Any visitors to your author's page should be able to purchase books directly from you if you are self-published or if you have permission from your publishers to sell the book yourself.
Create a PayPal account to complement your website. This account should be used to process and receive payments, as well as to make any business transactions that may occur while as you market your book online (paying for advertising, supplies, domains, services, etc).
Create a Google Adwords account. Bid on keywords that potential readers might type into a search engine to find information on the topic that your book covers. For example, if you write romance fiction, you might bid on words such as "romantic," "fiction," "books," etc. Ads should lead directly to your sales page. Of course, consider bidding on niche keywords that are not nearly as generalized as those mentioned above for the sake of avoiding fierce competition.
Market your book online with a one-on-one approach by setting up informational and slightly promotional pages on Facebook and Myspace. Make contacts with book clubs, authors, publishers, owners of books stores, etc. Do be careful not to overpromote to individuals who are using Myspace or Facebook solely for social purposes.
Consider writing for online publications that give authors' bylines. Even if you must write for free, promote your book online by offering your writing services to online publications in exchange for a quick endnote promoting your published work.
Join online book clubs, writer's groups, and community forums. Actively participate in groups that might be interested in your book genre. Be careful not to promote your new book in an offensive manner, and limit your references to it. Otherwise, you may be subject to penalties such as a deleted account by forum mediators.
Consider buying several domains to use for promotion. While www.yournamehere.com might generate a few hits, consider purchasing domains that will serve only to redirect viewers to your website. Of course, be relevant. Be sure that anyone using the domains you have purchased would potentially be looking for what your new book might have to offer.
Consider advertising directly on the websites and newsletters of groups with members who might be potential readers. Marketing a book online, especially if you are new author, means advertising directly in front of a target audience. Spend your advertising dollars well by looking for newsletters and websites that foster an audience for your book already.