More than you ever wanted to know about Unix, multiplied by ten. This book is absolutely stuffed from beginning to end with every sordid detail of Unix, from both a system administrator as well as a user perspective. Seriously, there is nothing this book doesn't cover in great detail. It's like 1,000 pages long. You'd have to be out of your mind to read the entire thing. It's great for reference and poking around, learning bits here and there. Everything is cross-referenced, and broken into small two- or three-page tutorials.
My favorite chapter was the one on sed. I had no idea sed was that powerful. My head was spinning the first time I read it. So I read it a second time, and I barely understood it all. What an amazingly powerful tool I had sitting on my computer all the time.
The one obvious downside of this book is its age. The latest edition is something like 10 years old now, and a lot has happened in that time. Some of the articles are irrelevant. There are lots of gotchas it points out regarding C shell, for example, before Bash sort of moved in and took over the shell space. It also talks about all the different Unix versions out there, but most of them nowadays are not very popular. Linux, FreeBSD, and Mac OS X (okay, maybe some Solaris too) are all that really matter any more.
But, damn, if you want to learn UNIX, you need this book, and you practically need nothing else.