Bickel's Mars Learning, although it doesn't follow up and Nagl since it came out prior to his work, does deal to a great degree with learning and applying lessons from COIN campaigns (although it's dealing with the Corps and covers the interwar period). Most of the stuff about Vietnam deals with either the COIN stuff or the major unit actions, and sadly I can't offhand point to a single work that deals with the applications of lessons learned and preserving those lessons in more depth than Nagl did. There are some from previous conflicts, though. Crossing the Deadly Ground springs to mind, although it does have a tight tactical focus. I'm sure there are others that just aren't coming to mind at the moment.
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