OpenBSD or FreeBSD. My PF kungfu is just enough to use it as a host firewall though. Recently I needed to level up my PF skill in order to use it on one of my hobby project. This book came in the right time.
Since I'm using OpenBSD, the excellent manual & FAQ at www.openbsd.org would have what I needed. But the commercial offering (of this book) illustrated PF differently and with much more working examples. This means my homework is already done mostly, leaving me to read and "realized" the knowledge. :)
This is a short summary on what I found interesting while reading this book: