In a nutshell: If you frequently work with the Mac OS X Terminal then you need this book. Not only is it a great reference for all your day-to-day commands, but it is also a book you can dip into and almost always learn something new and insightful.
As a software developer who has recently converted to using OS X as my main development box this book is proving an invaluable reference to working with the Mac Terminal/shell. I must confess that I only recently stumbled across this book after a recommendation by the author replying to a review I wrote for the Linux Pocket Guide (essentially the Linux-based version of this book). The aforementioned Linux reference has long since established itself within arms-reach on a shelf next to my desk, as I'm frequently referring to it when working with locally running Linux-based VMs or boxes running in the Cloud. Although it's still early days, I'm already thinking that this OS X pocket guide will earn a space on my desk right next to the Linux version, as it will not only save me mentally translating Linux commands into OS X parlance (where the syntax can sometimes be subtly different), but it's also a lot of fun to dip into during a brain-frazzle moment and learn a new command or switch.
Much like it's sister volume, this book is A5 in size and only 200 pages long, but it crams in an amazing amount of commands that should cater to all but the most hardcore of power users. It also has several bonus chapters discussing such things as installing the excellent Homebrew package manager (which is bizarrely missing from the default installation of OS X?). I'm assuming that most advanced OS X users will be aware of this already, but if you're just starting out in the world of power-Mac usage then this stuff will blow your mind! No more downloading obscure DMGs from the 'net and manually fiddling with the configuration options to get things running.
It could be argued that there are a few minor omissions in this book - I was a little disappointed not to see details on my beloved netstat and vm_stat commands, and I would have liked a bit more description on the output of some of the commands (e.g. top), but this really is nit-picking, and if you buy the Linux Pocket Guide at the same time as this book (which I would recommend if your serious about learning the shell) then you'll learn about these things anyway.
In summary, this is a great book for navigating the OS X shell, and whether you're a Terminal-novice or a seasoned power-user you will learn plenty (and most likely have fun doing it :-) ). Another great piece of work by the author.