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.
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.