Rusty Russell’s old iptables HOWTO was okay—or as okay as an iptables anything can be. An nftables HOWTO covering basically the same issues that The Book of PF does has the potential to be much better, simply because it would concern a much less messy system, but somebody has to write it. There’s little Linux-specific about this situation.
There's a culture of documentation in BSD which exceeds Linux. There's a larger commercial/reputational market for a Linux book of course, but that attracts different writers, who are often not as good/knowledgeable/motivated/dedicated/etc as the BSD documentation writers.
Additionally, BSD has a culture of simplicity and consistency which is entirely absent in Linux. This makes documentation more clear and much more durable.
