I've been interested in this topic lately. I think I'll buy this one. (PDF, $10). From an email:
... The wireless network security audit is, in fact, the last major section of the ebook, ...discussing common ways of restricting wireless network access that are akin to those bathroom door locks that can be picked with a paperclip ... show you how to encrypt your data in transit to protect it from prying eyes anywhere, which is particularly helpful when you're using insecure hot spots while traveling. Glenn also wrote a great section on securing small office wireless networks, complete with details on choosing VPN hardware and software, and on setting up 802.1X for secure Wi-Fi logins.
You can read more about the ebook, download a free 31-page sample, and place an order at:
We also just released the 1.2 update to "Take Control of Your AirPort Network," Glenn Fleishman's 161-page ebook about setting up, extending the range of, maintaining, and troubleshooting wireless networks with a focus on Apple's AirPort networking gear.... can save $2.50 by buying with "Take Control of Your Wi-Fi Security."