We use Perforce for source control here at work. Perforce is a little different than other version control systems I have used in the past (CVS, Subversion, VSS) so I decided to read O'Reilly's Practical Perforce to help bring me up to speed.
It was a huge help. Much like Open Source Development With CVS, this book goes beyond just command documentation and tutorials to really explain the Perforce view of the software development process. As I said, Perforce operates a little differently than other version control systems and learning Perforce's approach to the software development process really helped me understand why Perforce works the way it does.