At Alexa.com, click Traffic Rankings at the top navigation bar. Enter the address of the blog you want to evaluate and click Get Traffic Details. For most blogs, you'll see an Alexa rank from 1 (the most-visited site on the Web) to about 5 million, meaning very low readership. For the top 100,000 sites, Alexa provides detailed traffic estimates. Under the heading #Explore this site#, you'll see these links:
Traffic Details shows the blog's relative reach and number of page views, and whether traffic is trending up or down.
Related Links shows other sites popular with the same audience. Here you can discover more blogs frequented by your target audience.
Sites Linking In shows which sites, ranked by authority, have incoming links to the blog. Follow these links, and you'll find more sites targeting your audience.
Depending on how narrowly focused your book is, you may find only a few relevant quality blogs, and that's fine. It's better to focus on a small, well-qualified audience who will respond to your book instead of a general audience where you'll have little impact.
Alexa's reports aren't foolproof; they're drawn from a small sample of Web users who use its browser toolbar. Rankings for high-traffic sites are more statistically accurate than reports for niche sites. In any case, Alexa is a handy, free source of objective information about Web traffic, and is more accurate than anecdotal reports. Bloggers and Webmasters are notorious for overestimating their traffic.
Alexa, which is a subsidiary of Amazon.com, isn't limited to blogs, so you can use it to find all sorts of Web sites targeting your niche. Another good source of traffic estimates is #www.MetricsMarket.com#.