Hey,
I found these two sites in the forum, the former one being created by a forum member:
https://practicing-guitar.readthedocs.io/en/latest/
http://www.pianofundamentals.com/book/en/1.I.1
I play guitar for a while now and am practicing rather advanced stuff like Necrophagist and the like. I am not that good at it, but I tried to improve my overall practice efficiency over the past six months or so and found that the way HOW I practice is way more important than what or how much I practice. I also started to learn the piano very recently and I was wondering if there is any literature on how practicing actually works and what efficient methods there are to achieve good results in less time.
These two sites I linked are very good in my opinion, pianofundamentals goes into very much detail about efficient practicing. However, I think that the layout of this site is very confusing, and also, I am more of a paper kind of person. So I was wondering, is there any book which I could buy in actual paper form which is comparable?
I live in Germany, but I think I could handle a book written in English, not sure if I am able to buy it from another country, though.