Do you use FreeBSD regularly? Are these books useful reading for somebody who doesn't use FreeBSD?
If you are familiar with other Unix-like operating systems such as Linux, and you would like to use FreeBSD, then I think the FreeBSD Mastery books that I mentioned which I have physical copies of are useful also.
If you are not familiar with Unix-like systems at all then the FreeBSD Mastery books are probably not yet so useful for you.
The FreeBSD Mastery books are the most useful to someone who is already using FreeBSD in my opinion.
Michael Lucas can delve into depths without making it sound too technical IMO. Sure, you can skip chapters too (ktrace in my case) and continue with the next one if it gets heavy.
His books can be read by newbies and seasoned pros alike.
Yes. They'll prepare you to switch over to FreeBSD. ;P
In all seriousness, the ZFS books would be helpful if you use ZFS on Linux. The FreeBSD-specific books probably not so much. Those are the only ones I've read.