This book is on the pricey side (IMO), so I picked up a copy at my local uni library. As usual, you get what you pay for; for relatively savvy sysadmin types, this book will take your BSD-focused security knowledge to the next level.
I've worked a fair amount with FBSD jails, and still found some very useful tips within. Another valuable lesson from this book was a proper strategy for utilizing file flags and securelevels. HIDS, packet filtering, service hardening, etc. are covered in detail.
I own and have read several different Linux-focused security books, but this is the most comprehensive BSD-focused security title I am aware of. Definitely worth a read. (And if you're familiar with both FreeBSD and OpenBSD you will likely get even more value from it.)