Directing is fun. The only way to learn to be a director is to watch other directors in their work. As a result every directorial technique is different, and so there are few, useful, books on the actual methodology involved, but more on the process of organising a performance. Directorial technique should not be confused with practicioner thory, which deals with the role of theatre and elements involved.
In conclusion it is pointless to write about directing because it depends upon the personal approach of each person to a performance text.
It may be pointless, but I am going to write about it. The first thing a director has to learn is that there is no pre-set way in which a text has to be performed. Following stage directions and having a historical-naturalistic approach are options, not compulsory things to do. You can do Machbeth with Elisabethan costumes, or you can do Machbeth with Potato Heads. As long as your approach is saying something about the theme of your play, it's fine. Don't stage a play. Stage your play. And stage the play you'd like to see on stage. Then you'll see how other people like it too, and realise you were not so different and special...
To learn about directing you can do lots of things, but the first one is decide: I like this, I don't like this. Go and see as many shows as possible. Decide what you liked, why, and how that director got there. You must also get to know actors. The best way to do this is to either marry one or be one before you start directing. And you must know how actors work. How to get from people what you want them to give you. How group dynamics work, and how to 'unlock' them to make them work creatively. Experience can give you this.
The golden rule is: what do I like. why. how did that happen, how can I make it happen. This is applicable to lighting, spaces that you like, costumes, etc.
Good book: The Twentieth Century Performance Reader.