Re-learning or learning?

I recently considered choosing a chess book to improve and expand my chess knowledge, the book I chose was Simple Chess, by Michael Stean. The book seems to be well spoken and renowned in the world of chess books. It's apparently an introductory book to positional chess, but reading it left me with doubts. This is because I came into contact with strategic chess very prematurely, when I had around 1300 online rating (here on chess.com) and that was all I did to get to 1800-1900 (in addition to improving my tactics).

I've already read other books: Pachman's and Amateur's Mind, in addition to learning other important individual ideas. My strength in chess and my difference between most players in my rating range is that I believe I play better endings and positional play is my strong point.

The question is: to move forward, do I need to re-learn the material in order to concretize my knowledge or seek another type of information? recommendations?

it's been kinda tough.

After a 25yr lay off ... I'm relearning everything. I forgot everything. (currently about at half strength of what I used to be)

IMO Chess is like a language, if you don't use it you lose it.

The only thing I have going for me... is I know what to aim at with building a doable repertoire.

First time out I floundered in the repertoire dept. which stalled my overall game.

@1

Analyse your lost games and learn from your mistakes.
That will give you the clearest idea of what to work on.

Continue to improve your endgame technique by studying advanced endgame principles, complex pawn endings and practical endgame play.

Perhaps, you should change openings.

Play more positional lines or lines which try to trade queens early to get into endgames faster.