You can also find some material on the RSC in Wellington's Headquarters by SPG Ward. It is also being republished:

https://www.amazon.com/Wellingtons-Headquarters-Command-Administration-Peninsular/dp/1473896827/ref=sr_1_1?keywords=Wellington%27s+headquarters&qid=1551378761&s=books&sr=1-1

I agree with you on Wellington's Engineers. It is an excellent book.