I practice this for more than 10 years. Each day I work with computer for ~10 h., drive a car and do other things, and never wear glasses, even though the traditional ophtalmologic measures clearly indicate that I need strong glasses and I shouldn't see even the biggest letter on the Snellen chart, but I see not only the biggest, but sometimes even the 20/20.
Doctors can't explain this, and only congrats me on my achievement. Of course, the eyesight is not perfect. I see clearly in the daytime, but in the nighttime or low light conditions it becomes much harder to distinguish faces.
The best book I found so far is "Relearning to See" by Thomas R. Quackenbush. The originator of this theory was William Bates.
Actually, there is no clear unified theory on how to achieve this. Everyone interprets it differently and the results are inconsistent. There is also a lot of criticism from the medical establishment.
Natural eyesight improvement really works. And the unified theory, in a form of an app, or a good book, maybe including findings from neuroplasticity, would be a great gift for humanity.