Dave Taylor's "Learning Unix for OS X Mountain Lion" offers an in-depth tutorial for OS X power users who have not yet mastered Unix. Written for technically competent readers, it focuses on introducing common command-line tasks.
The book starts with Terminal, and then moves on to the basic forms of Unix commands. In just about 200 pages, it surveys common file tasks, application launching and even X11.
There are a lot of useful nuggets of information to be found. The tone overall is a bit dry, and I was rather hoping for more motivation on topics. That said, the author has done a good job in surveying Unix.
My favorite bits were the in-text notes that offered practical advice such as differentiating between absolute and relative paths, what noclobber is, and so forth. In those, the content got a lot more real-world, and the tone was a bit warmer.
It's a good book if you are, for example, a developer who's considering exploring the command line, or someone who's regularly responsible for system administration, but it's really not intended for a general Mac audience.
If you've ever wondered about permissions strings or wanted to access file systems directly, and need to basically understand what's really going on behind that lovely OS X GUI, Learning Unix for OS X Mountain Lion offers a basic introduction that will get you started. It retails for US$20 print, $12 e-book or $22 for both.