...for whatever reason, three books that I consider essential to any reading on insurgency tend not to be found on most "lists" I see published recently.

They are The Guerrilla Reader: A Historical Anthology, edited by Walter Laquer - published in '77, its a collection of essays and excerpts from larger writings on the subject throughout history. Its the companion volume to his outstanding book Guerrilla: A Historical and Critical Study, which was published the year prior.

The other book is Inside War: The Guerrilla Conflict in Missouri During the American Civil War, by Michael Fellman, published in '89. Not merely a history of the conflict, it offers insights that are of real value in the COE. When read in this context, I especially recommend the sections on Loyalty, Neutrality and Survival Lies and Collapse of the Sense of Security in the chapter on Civilians in Guerrilla War.