What books should I read to learn about Forex trading?

Day Trading and Swing Trading the Currency Market: Technical and Fundamental Strategies to Profit from Market Moves by Kathy Lien is a tremendous all-around book.

It gives you a strong introduction to the craft of Forex trading and takes you right on through to some pretty deep topics.
Currency Trading for Dummies by Kathleen Brooks and Brian Dolanbro is a great book for beginners. It provides a good grounding in Forex trading, familiarizing you with the terms and teaching you the ropes of Forex trading.