Chess theory...where to find

Here are almost 14,000 free chess books:
www.getfreeebooks.com/almost-14000-free-chess-ebooks/

Project Gutenberg also has more free chess books:
www.gutenberg.org/ebooks/subject/1677

thanks to you all :) And, as special: "Tongues trees, writing in the brook finds teaching in stones, good things everywhere."
Have a good night; see you all at the board :)

Chess is a game of two, this is why it's possible to make an elo estimation about the level where players play theory as the usual thing, talking about openings to complement previous posts.
Tip:
If you play against a noob you will never play mainlines but you will have room to play similar ideas as mainlines,
or simply put, good moves as well, following a plan
like centralizing the pieces.
Therefore it's not opening theory what you need at the early stages but the ability to spot weakness.
Chess is an agreement between two gentleman folllowing certain rules well thought with the continuation of their play such as tennis, in that agreement you see the confrontation between the two sides moving from one side to another and speed and accuracy are very well regarded.
The field of the game is the board that asks for a well rounded player with imagination, resistance agression stamina and tactical vision, space awareness, this is why chess is sport but as you can tell with practise it's more than sport its like baroque compositions, since the board is filled with pieces ( space supperpositions..)
only in the endgame it starts being less cramped but It still keeps a lot of posibilities and complexities
Since its very hard to understand this for anyone
its good to think about a deck of cards where the number 52 similar to 64, a two digit-number, its big but its not enough to explain the complexity, that is the difficulty of the game, the answer lies in shuffling the cards and
shuffling the pieces (combinatories), this is a good simple mathematical explanation of the complexity of the game, truth is chess behaves more like " cut-exponential" trees instead
of shuffling but It does not matter in this small post
or lesson.

Play a closed system by this I mean try to keep closed the pawn structure or play c4 and forget about all the e4 theory because its extremely demanding, its a tip and a guess so I can not back it with my own experience.
Positional chess its what I recommend for a player like yourself as open games have a lot of lines and forced sequences.
But I said c4 as white and as black what can I say just reply and hope for the best, given all the advancements of computer chess I think Caro-kann its a solid opening and It might be not as drawish as We think, It was used by
Karpov.
I play for fun but if someday I wanted to compete fide for example I would select a system such as the english opening and the Caro-kann and train myself in that task,
since today my mind is not able to keep fortitude
against this three together e4 e5, ( the double pawn
I think its called) e4 c5 ( the sicilian) plus the french.
I do not have enough discipline to improve as a e4 player and you need a lot as Its very easy to get carried away
with the myriad of positions or a guide, hire a coach if you have the chance a good one as I think you are very good already, @Anna-S-Fuchs .

as answer to "Blindfolded...": i learned chess by reading Tarrasch, Steinitz and Euwe. The guys at my first club was used to play sicillian and gambits. Sharp Gambits. We spent days by analyzing positions ans unusual positions ans uncommon outcomings of trade. You know, sacrifice a queen or a rook for a knight or something or sometimes a pawn. A book, which handels sacrifices at c/f/3 and c/f/6 was like a bible. U need to know it if u wanna play with those guys. None of these guys plays higher than 2. Bundesliga but they all play sharp and deadly if u fail.

I do not want to have a coach. I am old and i play for fun. They should take care about young people and teach them.
After te pandemic i will try to face WFM if possible and if i can relearn enough. But that is only a dream.

Have a nice day and thanks for your thoughts. Thumbs up :)

and also thanks to Borgdof for that. I will try that.