Jacek Artymiak, Building Firewalls with OpenBSD and PF, 2nd Edition (devGuide.net, 2003). Traditionally the recommended PF book since its first publication, it covers PF in OpenBSD 3.4 in great detail.

Michael W. Lucas, Absolute OpenBSD (No Starch Press, 2003). Written at the time of OpenBSD 3.4, this volume offers a thorough walk-through of OpenBSD with a wealth of hands-on, practical material.

Michael W. Lucas, Network Flow Analysis (No Starch Press, 2010). One of a select few books about network analysis and management using free NetFlow-based tools, this book shows you the tools and methods to discover just what really happens in your network.

Brandon Palmer and Jose Nazario, Secure Architectures with OpenBSD (Addison-Wesley, 2004). This book provides an overview of OpenBSD’s features with a marked slant toward building secure and reliable systems. The book references OpenBSD 3.4 as the then up-to-date version.

Douglas R. Mauro and Kevin J. Schmidt, Essential SNMP, 2nd Edition (O’Reilly Media, 2005). As the title says, an essential reference book about SNMP.

Jeremy C. Reed (editor), The OpenBSD PF Packet Filter Book (Reed Media Services, 2006). The book is based on the PF User Guide, extended to cover PF on FreeBSD, NetBSD, and DragonFly BSD, and with some additional material on third-party tools that interoperate with PF.

Christopher M. Buechler and Jim Pingle, pfSense: The Definitive Guide (Reed Media Services, 2009). At some 515 pages, this is a comprehensive guide to the FreeBSD and PF-based firewall appliance distribution.