When learning about networking and security, you'll see alot of references to RFCs which are documents that basically describe the operation and features of protocols, ports, standards and practices.  They can be rather cryptically written but with the right guide, you can easily find the information you need.  The RFC Sourcebook is this guide.  With it, you can quickly find out about any RFC or standard relating to TCP/IP, data encapsulation, PPP, etc.  It also contains nice reference guides to the stuff you never use but may need in an extreme situation of diagnosing a network problem, like ICMP codes.  Detailed information can be found regarding anything you can think of related to networking, encryption, authors of RFCs, organizations related to the computing world (like IANA, EFF, ICANN, etc), protocols, etc.

So if you've ever been in need of a quick reference for RFCs, I'd give RFC Sourcebook a serious look.