Ubuntu Unleashed 2011 Edition: Covering 10.10 and 11.04 (6th Edition)

http://www.insecure.org/nmap/ This site contains information on Nmap.

http://www.securityfocus.com/ The Security Focus website.

http://www.tripwire.org Information and download links for the open-source version of Tripwire.

http://www.ubuntu.com/usn The official Ubuntu security notices list; well worth keeping an eye on!