Peter N. M. Hansteen's PF tutorial and books are recommended reads, Peter remains involved with the developers and the information stays relevant and useful. He also ensures that readers using other BSD systems, especially with older versions of pf, can learn just as much from it.
* The Book of PF, 3rd Edition, 2014 - ISBN: 978-1593275891
Michael W Lucas is another author that writes books for both the BSD and sysadmin communities, similarly, he works closely with developers and users to release these short, yet all-encompassing tomes of information, covering a wide variety of topics.
* Absolute OpenBSD, 2nd Edition, 2013 - ISBN: 978-1593274764
* SSH Mastery, 2012 - ISBN: 978-1470069711
* Sudo Master, 2013 - ISBN: 978-1493626205
And of course, official documentation is great. The effort of many people working to improve, Jason McIntyre improving readability and overall quality, Ingo Schwarze's amazing work on mandoc(1) tools. OpenBSD's FAQ, which is usually the first step people take to learn more about the system, is maintained by Nick Holland.