"No Relief: Surveying the effects of gun violence on humanitarian and development personnel" is a study done by the Centre for Humanitarian Dialogue that examines the impact of violence involving firearms on aid workers. Some very good information and statistics. (Thanks for the reference Nick.)
The Monterey Institute of International Studies Security and Development program has also produced a few relevant studies. Specifically, Clinton Watt's "Indicators of NGO Security in Afghanistan" and Charlotte Savidge's "Insecurity of Aid Workers Engaged in Post-Conflict Reconstruction: The Cases of Bosnia-Herzegovina, Kosovo, Afghanistan, and Iraq." Both of these papers were published in 2004 and discuss issues from a perspective typically not found in field manuals and general training.
Academic study of NGO security practices is crucial to us "getting it right." It's all too easy to make decisions based on anecdotal data or what are historically believed to be truths. Having researchers objectively validate or invalidate our perceptions and cast light in areas we might not have a full understanding of, will help us enhance the safety of the people we serve.