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Kjell Jørgen Hole's Wi-Fi courseware (http://www.kjhole.com/Standards/WiFi/WiFiDownloads.html) is an excellent resource for understanding wireless networks. The courseware is mainly aimed at University of Bergen students who take Professor Hole's courses, but it is freely available and well worth reading.
For keeping up with developments in the wireless networking world, the WNN Wi-Fi Net News site offers running updates, with the security-related stories listed at http://wifinetnews.com/archives/cat_security.html.