A lot of Drupal books have come out lately. A ... lot. One that I've seen lately is called Drupal 6 - Social Networking and is actually pretty good. It is published by Packt Publishing and written by Michael Peacock. The goal of the book is to provide users who want to build their own social network (think Twitter, Facebook, ...) a step by step guide.
The book starts by explaining what a Social Network is and what it can do for you and your company. It goes then further by installing and configuring Drupal. It provides a nice walk through the most used configuration screen. After following that by introducing you to nodes, blocks, menus, themes and modules you get a nice overview of how Drupal is built up.



