I recently purchased a copy of the third edition of Learning the bash Shell by Cameron Newham and Bill Rosenblatt.
Those of you who know my background know that it's been quite a while since I learned my way around bash, so it was for other reasons that I bought this book, namely that I regard it as one of the better tomes on shell scripting in general and therefore just something I feel I need to have on my bookshelf.
This edition of the book covers the most recent major release of bash, 3.0, and therefore includes an appendix on programmable completion, which I proofread for Cameron, a fact that even scores me an acknowledgement in the preface. Seeing my name in print always appeals to my vanity.
In my experience, the vast majority of system administrators still haven't discovered or grown accustomed to the features that bash 2.x gave us, and that was some eight or nine years ago. How many sysadmins do you know, for example, who make use of the arrays introduced in bash 2.x?
Anyway, if you've only ever half-learned shell scripting by picking up bits and pieces of knowledge here and there from colleagues on the job, this book could be exactly what you need to unlock the full power of the tool. If you can learn how to wield all the power encompassed in this book, you'll be a very adept shell user, indeed.