Traceroute is an important network troubleshooting tool. Here's an excellent document that talks about traceroute in details.
This article gives a very good insight about how to use VLANs with Linux. I am sure most virtualization administrators would find it useful.
VLAN support in Linux
An excellent introduction to tcpdump. This is one tool, that all existing and wannabe infrastructure administrators should be well versed with.
Here's a very nice article that describes how develop a contingency plan. The article is not specifically focused on disaster recovery from IT perspective. Its more of a generic article and touches various points from the planning & management perspective. Although the article was published in 2001, however I think most of the points described will always hold true.