I would recommed pretty much any Unix/BSD book from O'Reilly, they have been usefull to me for years.
For anyone wanting to find more details on some of the well known Unix tools, e.g. sed/awk/grep etc you can get small O'Reilly's that go into great detail.
Also, since it's Unix, there is a heap of on-line documentation available as part of the OS.
I'm refering to the _man_ual pages. For pretty much any Unix utility, try man <command-name>
e.g. man grep
The man pages unfortunatly won't be there for OSX specific tools unless Apple is kind enough to right them, but it certainly provides a handy quick reference to commands.
p.s. OSX rules - I've been using about 10 different Unix's for around 6 years now, and OSX is the best OS I've ever had the pleasure of using...
