I think there are 2 answers to your questions here. Technique isn't just about finger strength, it's about the height of your stool, it's about the touch point of fingers to keys it's your ability to split dynamics, to vary tempo, to voice melodies, it's making sure your shoulders are comfortable, your wrists are firm but flexible, all these things can not (atleast to my knowledge) all be taught from a book. Whether a child prodigy or a casual keyboard player the simple fact is Everyone (feel free to correct me) at some point in their lives have used a Piano teacher. technique is as much as instinct as it is taught, in that if you are taught the right ways to begin with, it will become instinct. The balance of correct "technique" as built in instinct that you are born with is as much down to genes as it is chance as it is observation as a young child. My answer with regards to technique if this is your issue is - Get a piano teacher. Now your second question with regards to scales/left hand etc, this you can certainly find books for, there are a vast amount all that instruct differently, and teach methods differently, the key would be finding the right ones for you. There's a free book here called Mastering Scales and Arpeggios - https://imslp.org/wiki/Mastering_the_Scales_and_Arpeggios_(Cooke,_James_Francis)It's a little detailed but it breaks everything down for you, from where scales originated, to keys, to what we used today, to how to play them. it's a good read, have in mind that all piano practice needs to be done at the piano. With regards to your left hand, that simply comes by playing pieces that present new problems but are manageable, playing scales, arpeggios, and possibly even short studies *whispers* such as Hanon or Czerny studies that purposely aim to provide you riffs and melodies to develop strength in fingers and challenges you are likely to encounter when learning pieces. Mostly of these again are free - to google Czerny Studies or Hanon studies will provide you books you can work from. I will say with all of this information, have a sensible study plan, don't pick up a book with the aim to blast through it in a week, then develop tendonitus and break your wrist for the next 6 months, there is simply no shortcut to playing ballads, Chopin Etudes, etc realistically you're a good few years away before looking at those pieces based on your current performances. I'd definitely say i'm in a similar boat to you and happy to discuss anything further or work with you. I am incorporating various studies and pieces into my daily practice that's working to be efficient for me without taking too much time (I also work full time in IT) I'll end by referring you to my original point however, - Look at your options for a piano teacher. At the very least post some of your performances up here in the audition section, doesn't have to just be finished stuff, things you're working on, there are many teacher on this site that can have a look at both how and what you play. Hope that helps.