Full disclosure - I've owned a copy of "Unix in a Nutshell" for over 20 years, since the 2nd Edition (1993).
"Unix in a Nutshell" is the epitome of the O'Reilly "In a Nutshell" series. It is the benchmark I compare all other Nutshell books to - some come close, others miss the mark.
The key portions of this book are:
* Alphabetical Reference of commands
* Shell commands for Bash, Korn and C-shell
* Overviews of Sed and Awk
The other chapters are important, but are more specialized beyond general user or intro sys admin work and I haven't had much use for them.
For Unix die-hards, the first complaint I read/hear about this book - "Why not just use the Man pages?" For those who cut our teeth on multiple Unices (Unixes?), the differences between the OS versions would require a shell script and 'diff' to figure it out. The command reference is NOT all inclusive for either all commands or their options - only the most common ones are listed, with a brief output of what will happen. Genius!
Man pages have their uses, and if you want to be staring at a computer terminal for hours, go right ahead. But this book, in concert with man pages and online references, can give you that "multi-track attack" in learning how to use your Unix system.
One downside to this tome - every edition gets larger and the price has gone up from $15 (in 1992) to around $50 (2014). While I love this book, the price has precluded me from giving it as a gift to friends/family who use Unix. If your wanting specific topics, some of O'Reilly's Pocket Guides (around $15!) may hit the mark.
If you want more Linux specific information, purchase "Linux in a Nutshell"; around the same cost, but less Unix agnostic. And if you're waiting for "FreeBSD in a Nutshell," keep waiting...