I've been a Unix users (in various flavors) for over twenty years. I had heard about BSD in the early 2000s, but I could never find a copy of installation media (disks, CDROMs) to get it started up... until 2008. After I obtained a copy of FreeBSD 7.0, I've kept a copy on at least one system in my inventory... but not until I read Dru Lavigne's book "BSD Hacks" did I actually unlock some of the potential in that BSD box.
The learning curve is steady throughout the book, allowing you to tweak your system with Dru's hacks. A lot of the security related ones deal with OpenBSD or OpenBSD-derived commands on FreeBSD. It was a great read first-time through, and a irreplaceable reference for my BSDing.
The only downside is... since O'Reilly has seemingly discontinued the "Hacks" series, there will probably not be an update for this book. Some programs mentioned with (CVS comes to mind), has been deprecated in BSDs, and totally removed in current versions. So those hacks fall flat, now. Others just require tweaks from the older code/commands to newer versions.
BSD-derived variants took a Public Relations hit in the early 1990s, with the AT&T/Univ of California Berkeley fight... hence the reason Linux leapt in popularity. Dru's book helped drag BSD out of obscurity for a Unix-lifer like me... and if you get a (used) copy, it can do the same for you!