To keep a tight focus on Firefox in this book, I cut the Thunderbird discussion short here. However, if you like what you see so far, here are three great (and free) online resources:
Official Thunderbird Help: The Mozilla Foundation's official Thunderbird support page, offering links to a Frequently Asked Questions (FAQ) page, free online support forums, and a list of tips and tricks. You can reach this page easily by choosing Help Mozilla Thunderbird Help in Thunderbird.
An Introduction to Thunderbird: A wonderfully thorough, screenshot-laden online walkthrough of Thunderbird, beginning with installation.
Harvard's Thunderbird Help Center: Offers helpful tutorials for each of Thunderbird's primary features, with screenshots.