Like Tim Donahue said, you can’t learn how to act from a book. But they can give you some ideas to carry with you as you learn.
The Stanislavski is possibly “The” acting book to read, particularly when it comes to The Method style of acting, although I found it a bit tough going and sometimes made little sense to me. But then it is a translation of an 80ish year old Russian text. [Correction: A couple of days later and I’ve realised it was written in English, not Stanislavski’s native Russian.]
One evening course I did had everyone read Uta Hagen’s Respect for Acting, which, if I recall correctly, was a reiteration or elaboration on Stanislavski’s ideas. It also has a bunch of simple exercises she says you should practice a lot on your own, which may be useful for some people - I found this style of things a bit too “interior” for me.
I really liked Sanford Meisner On Acting. He moved things on to encourage the actor to focus on the person they’re acting with, and to encourage the ability to respond to them naturally. This made a lot of sense to me. I think there are DVD(s) of him teaching available too.
My notes on those three books are on my website, which might give you a flavour of them.
Then there are very different styles of acting. I did a course based on the teaching of Jacques Lecoq, which is more physical, perhaps less “thinky”. His book The Moving Body might be an interesting contrast to those above. Some people on that course seemed very keen on Grotowski’s Towards A Poor Theatre, as well as Johnstone’s Impro, which you mention and is often raved about. Good luck!
Phil (ex-acting student)