Absolute FreeBSD 2nd Edition is THE FreeBSD book (along with the handbook of course).
The Best of FreeBSD Basics is also nice but it's completely different from Lucas' book, it's tip/tasks oriented while Absolute FreeBSD is very deep, an unique reference, extremely well written, explains theory and is simply the FreeBSD book written so far.
Building a Server with FreeBSD 7 is also very nice. Nice surprise and worth every penny.