Do all Unix Books apply to FreeBSD?

Hello FreeBSD, I want to learn more about FreeBSD and my question is since FreeBSD is unix-like and cannot use unix because of licensing. When I buy a book about UNIX does all of the commands in the book work in FreeBSD?
FreeBSD is UNIX-based so much of the content in other UNIX books will relate to it (as will Linux since it is UNIX-like). However if you are interested in other UNIXes you shouldn't need to license the UNIX brandname yourself to use an OS. Just pick up a second hand server running Solaris or (slightly pricier server running AIX).
Very quickly, O'Reilly "Unix in a Nutshell" is a good reference book to use when learning basics. They published a "Berkely Edition" 30 years ago, which is more specific to FreeBSD than the later 3rd and 4th editions. However, since FreeBSD has many System V/Solaris features available, much of which is in the 4th edition is applicable.
A very valuable adjunct is O'Reilly "Learning the vi and vim editors." The major reason for knowing vi is that it has been the basic administration editor that is in base system installations of every "Unix" distribution made since about 1980. vim is a clone generally standard in Linux. You may prefer emacs, but it is not always installed initially.

Don't overlook the value of man() pages. Those are installed on your system, and are the most specific information about what is actually installed on your system.

The above are things to get you going if you are new to Unix.

For greater depth, you can read Marshall Mckusic "The Design and Implementation of the FreeBSD Operating System."