Day Trading and Swing Trading the Currency Market by Kathy Lien thoroughly discusses the salient parts of currency trading. I consider this book number one currently in the intermediate-level forex trading category. The reason I rate this book number one is because Kathy Lien is a leading expert in currency trading and her writing demonstrates her expertise.
Kathy talks about what moves the currency market in the short and long term, how to use currency correlations in trading, seasonality, technical and fundamental trading strategies, and more. Each chapter, in general, is comprehensive in scope and yet is written clearly for ease of understanding.
I especially like the chapter on currency correlations. Although currency correlations need to be recalculated at least monthly, there are numerous ways in which a creative trader could take this information and turn it into a trading system or use it as a filter in a trading system.
All in all, Day Trading and Swing Trading the Currency Market is an outstanding book that can be used as a resource to be reread as needed.