What fascinates me so much about Itten's classic book is its deeply philosophical, even quasi-religious approach to color. Itten was as much interested in the spirituality of color as in developing a theory of color.
Another great aspect of the book is the numerous master paintings beautifully reproduced and intertwined with his own theories and color diagrams. You get paintings by Rembrandt, Renoir, Monet, El Greco, Seurat, and so many others. Itten is always contrasting the objective and the subjective aspects of art and color, and it makes for a fascinating reading experience.
Itten has a mature perspective on these painters and their relation to color. Its only flaw is that Itten's is only one perspective. I wish there were more authors who could write and analyze art at this level, and who weren't afraid to be spiritual in their assessments.
With so many dramatic advancements in technology, we tend to forget very quickly where we were. Just a decade or so ago, this book was literally too difficult and too expensive to produce. We are lucky to live in an age in which a book of such profound beauty is even available. Overall, it's a must for any fine library.