Absolute FreeBSD by Michael Lucas is the one I'd recommend these days. In the past I'd recommend The Complete FreeBSD, but it's out of print and it hasn't been updated in years AFAICT.
And not strictly FreeBSD specific, but The Book of PF by Peter N.M. Hansteen is great for learning how to set up that particular firewall. And you might consider Automating System Administration with Perl, Second Edition by David N. Blank-Edelman
Apart from The Complete FreeBSD those are all available from O'Reilly in both paper and electronic versions.