A Classic, Reissued
David Galula, a French army officer with extensive experience in counterinsurgency, wrote this classic in 1964. The book was rediscovered in 2006, as was counterinsurgency itself. Most of the contemporary doctrine -- that the population is the prize, that a gun is not always the most effective tool -- is found in its 99 pages, and was there all along. It is cited in the bibliography of every work I've read on the subject since then.
The 4-page foreword by John Nagl is an even more succinct summary, and worth the price of the book by itself.