I would consider insurgencies to be an outcome of a political problem rather than a doctrinal one (although it does present a series of unique doctrinal problems). Insurgency is related to war; while depth etc is related to warfare - subtly and profoundly different concepts (in my view, anyway). Regardless the more astute, knowledgeable and articulate members will hopefully shed more light on this matter than I can.
As to suggested further readings:
On Infantry, by Bruce Gudmundsson & John English:http://www.amazon.com/Infantry-John-.../dp/0275949729 - information about the tactical application of defence in depth. Very good in the early evolution to the 'elastic defence' in WW1 (Gudmundsson's Stormtrooper Tactics deals with this period of history and move to mobile defence in more detail, if you're interested).
Military Power, by Stephen Biddle:http://www.amazon.com/Military-Power.../dp/0691116458 - a very confident, very interesting framework of a 'modern' military system that includes defence in depth, outlining how and why it works (in Biddle's view this 'modern system' has being in place since 1918). You will need a post-grad qualification in maths to progress into the second half of the book, though, but the first half is worth the shelf price alone.
Operational Art, Centino (see Post 5 below for details & link) - a very good and readable account of campaigning in recent times. Centino includes a lot of theatres you would otherwise be pushed to read about - India and Pakistan, for example - and launches a rather blistering (and amusing!) attack on manoeuvre warfare.
Bookmarks