A steady audience on your Web site provides additional income opportunities through affiliate programs and advertising. If your site becomes extremely popular, the revenue could rival your income from book sales.
Some bloggers report that a combination of affiliate and advertising revenue can result in about 1.5 cents of income for each unique daily visitor to your site. At that rate, a site averaging 1,500 unique daily visitors can generate about $8,200 in annual revenue--not bad for something that requires no ongoing work on your part. Depending on your audience and the type of products related to your book, you might do better or worse.
New sites usually generate negligible revenue, but advertising or affiliate programs can still be worthwhile. Your audience may appreciate niche advertising, and these programs can boost your visibility with search engines. One option is to donate your affiliate and ad revenues to charities admired by your audience, which sometimes can be handled automatically. The public-relations benefit of donating could outweigh the monetary value, and you won't have to account for it as income and pay tax on it.
In any case, advertising shouldn't overly distract visitors from the main purpose of your site--generating awareness of your book.
Here are some of the leading advertising and affiliate programs authors can use on their Web sites: