As the copy on the back of this book claims, the O'Reilly 'Bash Pocket Reference' allows one to "quickly find answers to the annoying questions that always come up when you're writing shell scripts." As you would expect, this is a reference manual, and unless you already know the fundamentals of BASH, it won't be of much use, unless used in conjunction with another, more expansive book. My favorite part of this book so far has been its treatment of more obscure shell variables, and its quick reference for shell builtins.
I should point out, though (and this is something that a previous reviewer seems to have missed), that this guide does *not* cover the use/syntax of all the external programs one might find on a *NIX system (e.g., grep, find, etc.). The reference is to BASH only and its associated builtin commands. And to be honest, external commands don't really *need* a reference like this, since they should already have man-pages that explain their usage.
Considering the price and the abundance of information, the BASH pocket reference is a great buy for anyone who does even casual work in the BASH shell.