I want to be able to read a non-technical book fully and I can't and I'm very unhappy about this whole situation.
It's surprisingly easy when forced. I moved apartment a few years ago to a small seaside town (mostly retirees and a few locally run shops). We had an old CRT style TV with very few channels. For the first two weeks we had no internet. It was pretty interesting how our differently we would spend the day. My friend and I would take walks around town or the beach, go surfing for a few hours, cook instead of ordering takeout, and then spend the rest of the time hanging out in the living room. The TV was on but we were mostly just chatting. Switch to two weeks later when we got internet. We spent much more time in our rooms on our computers playing video games or watching Netflix. We would still surf a couple of times a week but besides that we would only hangout for an hour or two in the evening if there was a good movie on TV. As frustrating as it was at times not having internet access I would say I was much happier during those first 2 weeks and with so few options for things to do my attention span was automatically much better.
Your message started with "What wouldn't I give to have such attention span!", and it turns out that you don't need to give all that much. Getting started is difficult, though.
If you do start down this path, consider reading Deep Work by Cal Newport to give you some ideas about how you can use the focus and concentration abilities you're building. Again, it's not perfect, but if you take the book as a list of suggestions rather than a strict prescription of what you should do, you'll probably find that you can adapt its lessons to make your life better.
Work on engaging in those activities a little bit longer then you currently can, and slowly increase the duration. Use a timer to set a limit. This would need to be done with regularity and focus.