To Learn More
Network security is a rapidly changing field, but there are many current works and some classics everyone should read. Garfinkel and Spafford is an excellent place to start. For depth on hackers' tools and techniques, read Scambrey et al. Some of the early works on network security have been reprinted in Davies and Price and Abrams and Podell.
Anderson presents a well-thought-out approach to putting the pieces together securely. Schneier presents the concepts of network security clearly.
For firewalls, Cheswick and Bellovin and Chapman and Zwicky are recent revisions of two standards. For intrusion detection systems, Northcutt is recommended; Allen and Kemmerer and Vigna also give good surveys of the state of the field.