When I was a teenager, I subscribed to several guitar-oriented magazines, each with pages of ads in the back of every issue, selling everything from picks, to amps, to guitar instruction. I was always trying to play faster and more complicated pieces, so when I saw an ad for a "finger exerciser," I had to have it.
A couple of weeks later, a package arrived in the mail. I'm not sure how to accurately describe what it contained other than that it was a combination of Velcro rings you attached to your fingers and rubber bands, which provided resistance to work against.
At the time, I was taking weekly guitar lessons. I remember telling my teacher about my new purchase and how it was going to help me become a better player.
I'll never forget his response.
"If you want to get better at playing guitar, play more guitar."
Forget the theory. The most valuable classroom is the real world and the only true learning happens when you actually do something.