Thanks for the reply - Indeed an extensive list of books.
Interesting note on Maj Cocks, in view of W's reluctance to formal training, your comment "...being groomed by him for higher things." is intriguing. While not new knowledge, it always trigger some thought about W's apparent lack of trust in people outside his immediate sphere of control. It may also show something about his ability to train other officers himself (in modern leadership terminology it would probably be called mentoring or coaching). If you look at some of the other officers of his HQ (QMG George Murray, MS Fitzroy Somerset, and (your own) Scovell, to name but three), there seem to be a pattern of increasing trust, probably or maybe based on a demonstrated ability over time.
That the Duke of York strongly supported Egerton's products is, as you say, another pointer to his understanding of the necessity of a theoretical base for the oficers.
I would also guess the Le Marchant had some saying in this support, as the RMA, after a few years of activity (five in 1804?), probably had enough experience to define in some detail their need for military publications.