The FreeBSD Handbook - It covers everything from installation and UNIX basics to jails (like containers), security and networking.
The FreeBSD manpages - The UNIX tools that you might be use to on Linux can sometimes be slightly different. BSD distributions do not use the GNU version of that ship with most Linux. Check out the GNU version and the BSD version of the source code for the
The BSD Now Podcast - The hosts go over new features in the popular BSD distributions, have interviews with maintainers and developers, and answer listener mail. They also have a growing list of tutorials on their website to read through.
I hope you consider installing PC-BSD or at least spinning up a FreeBSD droplet on Digital Ocean and playing around with BSD. Even if you are a die hard Linux user, experiencing the differences could lead to a better understanding of Linux.
Note: The link to Digital Ocean is a referral link.