Hilary's book has updated information on lightfastness, handling, etc. for various pigments, but I still recommend the Wilcox book. I have both books in my library. This book was reprinted in 1993. It's a definitive work on the relationship of design elements and principles to nature, with profuse illustrations and exercises to sharpen your perceptual and analytical skills. The focus of this book is on design, and taking advantage of the fluidity of watercolor and its accidental effects .
Techniques emphasize back or negative painting and glazing .
This is a marvelous book if you like to experiment with different mediums, or if you feel like you need a push in a new direction and don't know how to get started. Edward Betts is a renowned artist and teacher. A good book for those interested in multi-media painting. Discusses characteristics and techniques for watercolor, gouache, acrylic, and casein. Lots of colored pictures.