I'm generally a fan of Dru's books/articles and of O'Reilly's various Hacks books. But this is a notch above the rest with a lot of practical tips.
This was the second BSD book I owned, and it is the first place I learned about
- Some simple, but highly useful CLI navigation tips for tcsh (which, incidentally also happened to apply to bash).
- Creative ways to use grep and find.
- Using mtree (part of the FBSD base system) as part of a HIDS deployment.
- The very handy g4u cloning application.
Some of the hacks may be a bit dated as of this writing. But still a very cool book, and worth the (modest) price.