I know this is pretty dated (1998!) and the colors hurt your eyes some. But read Don Rae’s essays if you want to be a more competitive player. And when you’re done reading them, read them a 2nd time. Come back to them after a couple of weeks of applying his ideas and then read them again!
http://donsessays.freeservers.com/
He wrote those five essays way before there was anything but A&A classic. So not everything is applicable anymore (we have new units he didn’t write about and the values have changed for some of the others he studied) and some of it’s just written towards the classic map. However, I have yet to find a series of essays as well thought out and written as his are. Other essays and writings I’ve seen have talked about openings, recommended moves & buys, etc. But not too many go deeper into WHY you should make certain moves/buys and why others aren’t so great.
You should really key in on when he writes about deadzones, infantry push mechanics, and setting up stable fronts. The infantry push mechanic has changed some since you can now get mech that move 2 spaces. But the basic premise behind it hasn’t changed, protect your valuable pieces!