Because of their interactive features and visibility on the Web, blogs have largely replaced other book-publicity vehicles. Just a few years ago, "chat" sessions on proprietary services like America Online were a prime target of book publicists. Although the audiences for many of those proprietary services are now splintered across the Web, a whole world of opportunity remains outside the blogosphere.
Popular Web sites devoted to a certain topic or book genre often have a lively discussion board. The best way to find these discussion groups is to search Google for the types of questions your audience tends to ask most frequently. Once you've found them, contribute to the conversation, but don't blatantly advertise your book. Include a discreet signature at the bottom of your posts, three or four lines of text listing your Web site, contact information, and buy-the-book links.