Packt has two e/books, one for using FreeBSD as a server and another for FreeNAS.
Sam's Unleashed had a good book, which is now very outdated, and now only available in ebook form.
Many here may point to Absolute FreeBSD. I tried a different book from No Starch Press, about PF firewalls, and it was adequate, but a little difficult to sort through.
Go to different publisher's websites and do searches, some based on the same publishers you are already familiar with.
There aren't many books for FreeBSD as there are for other operating systems, so perhaps, you can look for books on specific open source server software.
* edit - check out O'Reilly publisher too.