Assuming, though, that you are looking for great-great-great Uncle Jed lost somewhere in the mists of the Ozarks in the last couple of centuries, it's hard to imagine a book that could be more useful for telling you how to track him down online. If you know about computers and are an experienced genealogist, skip Part I: The Basics, which takes the reader through modems, access options and choosing an ISP; browsers, FTP, e-mail, and Web publishing; and how to begin looking for your ancestors, including standards, sources and proofs, and sharing information. Internet skills encompassing Usenet, mailing lists, chat, FTP and search engines are covered in Part II: The Internet, and even if you are at ease on the Net, this section would be useful for its focus on the genealogical side of such online entities.
Most interesting is Part III: The "Must See" Online Resources, whose near-170 pages cover in exhaustive detail an array of Internet facilities and Web sites related to genealogy, illustrated with screenshots and enriched with examples, instructions and first-person testimonials. The chapter covering Online Library Card Catalogs has applications that extend beyond genealogy, although, of course, all the examples are built around that topic.
There's no guarantee you'll find Uncle Jed, but this book should certainly point you in the right direction for looking.