Having reviewed Howard's upcoming book on Quantitative Trading Systems, I find it to be an excellent reference for anyone using AB. He really exposes the reader to a lot of trading system ideas, mixes in the appropriate statistical interpretations and uses it to provide guidance (some of which will change your thinking about trading system development) and includes a lot of sample code. The latter will introduce a lot of "neat" ways to use AFL and help you put your ideas into code.
I highly recommend this book to all.
