Manuals (CIA and NGO)
At the top of the list is the CIA's Psychology of Intelligence Analysis by Richards J. Heuer. This is a must read if you're into critical thinking and the inner game of security. It covers information gathering, analysis and the various biases that can creep in and influence decisions. The content presented in this manual should be taught in every introductory humanitarian security course.
Next up, Charlie writes in with some links to a few humanitarian security manuals and resources:
(A few of these may seem familiar, but it's good to mention again for new blog readers.)