Lupton and Phillip's Graphic Design was a great high-level overview to the field, offering a number of design exercises across sections of design: typography, color, pattern, layout, etc.. I've not had a chance to experiment with all of the exercises outlined, but I did do some letterform patterns which were a lot of fun.
I found some of Itten's explanations to be too basic for my needs (having already studied painting and read Josef Albers' Interaction of Color). But, the book is quite short and does contain worthwhile diagrams and explanations. One passage was particularly amusing, where Itten illustrates the subjective and psychological effects of color by asking readers to imagine a dinner party where meat, vegetables and potatoes are served to the guests, and the host alternates between flooding the room with red, then blue, then yellow light. Each hue alters the appearances of the meal's constituent parts, and it is only when the host stops his shenanigans and applies normal light again that the the guests regain their appetites. The lesson is an important one - various treatments applied to the same material will cast that material in a vastly different light, quite literally.
Another important passage pertains to the 3 types of "attitudes to color" among painters: 1) the "epigoni", who have no internal understanding of color and merely copy their teachers; 2) the "originals" - those who compose works according to their internal sensibilities, and you can see their color preferences applied throughout their body of work; and 3) the "universalists", artists who are more objective with color, applying the proper treatment to each individual subject matter. He remarks, "That there should be but few painters in this group is understandable, for their subjective timbre must comprehend the entire color circle, and this happens rarely. Besides, they must possess high intelligence, admitting of a comprehensive philosophy." I think we all aspire to be in the 3rd category.