1. The Bash Man Page
Yeah, I know. I spent nearly half a page in my book trashing the bash man page for its impenetrable style and its lack of any trace of user-friendliness, but nothing beats typing "man bash" when you're already working in a terminal. The trick is finding what you want in its enormous length. This can sometimes be a significant problem, but once you find what you are looking for, the information is always concise and authoritative though not always easy to understand. Still, this is the resource I use most often.
Perhaps in response to the usability issues found in the bash man page, the GNU Project produced the Bash Reference Manual. You can think of it as the bash man page translated into human readable form. While it lacks a tutorial focus and contains no usage examples, it is much easier to read and is more usefully organized than the bash man page.
3. Greg's Wiki
The bash man page and the Bash Reference Manual both extensively document the features found in bash. However, when we need a description of bash behavior, different resources are needed. The best by far is Greg's Wiki. This site covers a variety of topics, but of particular interest to us are the Bash FAQ which contains over one hundred frequently asked questions about bash, the Bash Pitfalls which describes many of the common problems script writers encounter with bash, and the Bash Guide, a useful set of tutorials for bash users. There are also several fun to read rants.
Like Greg's Wiki, the Bash Hackers Wiki provides many different articles relating to bash, its features, and its behavior. Included are some useful tutorials on various programming techniques and issues with scripting with bash. While the writing is, at times, a little chaotic, it does contain useful information. Heck, they even trash my Writing Shell Scripts tutorial (Hmmm...I really ought to fix some of that stuff).
Chet Ramey is the current maintainer of bash and he has his own page. On this page, you can find version information, latest news, and other things. The most useful document on the Bash Page is its version of the Bash FAQ. The NEWS file contains a concise list of features that have been added to each version of bash.
There you have it. Enough reading to keep even the most curious shell user busy for weeks. Enjoy!