I actually have the Book. It is very good however, if you read the handprint.com site you find that it has a couple of flaws - however, if you are having a hard time understanding the handprint site, it would help if you read the "Blue and Yellow don't Make Green" first. The best info is free though. Handprint.com, then when you really understand it and can use the simple secondary color wheel he recommends, go to http://www.realcolorwheel.com/tubecolors.htm
That info is all there free, even though you can purchase it too.
This is amazing that you can get all of this information free. I have spent months on handprint.com. I have used the secondary pallette and learned to mix all of my colors from it - it really works. I only buy watercolors for convenience and out of laziness. I does help in any medium that you work in though.