improving ?

It's mostly about following key principles when playing. That helped me reach 2000+ and that is what I teach others to help them do the same.

Depends on what rating I was and what is meant by "strategy."

If generalized then self-analysis with the chess.com analysis board after every game (especially losses) teaches the most, but if instead "strategy" is meant similarly to positional chess, then studying pawn structures and Jeremy Silman's Imbalances is the way to go if you are over 1600 chess.com level.

If below this, then getting introduced to the concept of outposts and weak squares can do a lot.

For over 50 years, I have recommended three books by Irving Chernev: Logical Chess Move by Move, The Most Instructive Games of Chess Ever Played, and Practical Chess Endings. When I give lessons or just advice to friends, I tell them that these books probably helped me more than any other.