In a good written Forex book you can expect to see either the theoretical background, or some success stories. We all know there is no book that can include all the angles and views about the Forex market, but you can get different perspectives and step by step, you start putting all the pieces together.
Any book goes hand in hand with the other side of your education process, that is web demo trading and/or attending a course. In a Forex book you might go into more details on what is behind some concepts. For instance, if you are interested in trading options on forex and want to learn more about the Black-Scholes model, its more likely that you will find the details in a book, rather than in a 2 or 3 days course or seminar.
A book can go in the probability theory, market volatility, and you could also find some mathematical approaches. It might help you to have a broad picture, and it is not that complicated as it may first sound.
On the other hand the are Forex Books that are centered on a success stories or are more focused on you as potential successful trader. The main message is that you can do it yourself! Take for instance The Way of a Turtle written by Curtis Faith, that is a famous book about a group of people with different social and educational background, who get trained in forex and finally they get millionaire. Why not thinking about yourself that you can do it too ?
Other authors view the learning and developing process in the forex area as a coaching process. This is the case of Brentt N. Stenbaarger who wrote The Daily Trading Coach that provides 101 lessons for you to become your own trading psychologist. You can intuitively see that at the beginning of the process you will need a master to assist you with all the ups and downs you will face. But at some point it is important to become a master yourself and manage your emotions, your impulses, to improve your forex trading results.