I believe that the mastery in forex trading can be achieved only through a lot of practice.
It goes without saying that it is hard to practise without any background knowledge and some theory. You asked for the advice of some ebook, but I personally think that most of the most renowned ebooks are outdated. However, what I like forex market for is that the market is well studied, so there are many credible educational resources which are available online and free of charge. For example, one of the most simpliest and plain resources concerning the topic of trading currencies is the websire babypips.com. There is lots of information which will be enough to start your journey in forex trading. The information there covers different aspects of trading and it is down to earth and practical.
So, I really feel that this knowledge and practce will make you understand how the whole thing works. In addition to that I should say that some decent part of trading is done almost automatically by experienced traders. I remember my first trading experience, where I didn't have any habits of tuning the indicators and I even had to search for the right button in order to do this or that trade. That where practice becomes important: you've got to get used to the trading platform in order not to ask the questions that you shouldn't ask, like some technical stuff.