Leatrice Eiseman has put together a beautiful guide to color usage that is useful to designers and non-designers alike. While it discusses color theory and the connotations of specific colors, I find its strength lies in the color combinations and the way they are categorized.
Many projects requires a solid color set, to convey a specific tone (subdued, traditional, unique…) – a daunting task while staring at a blank screen. Yet, easily solved by flipping through the book and choosing a few color sets for application within a layout.
For non-designers, this book is just as useful in other areas, including paint choices (why stick with boring wall colors?), and I would assume, fashion.
While this book has much in common with Color Index, I think of them as complimentary to each other, as opposed to redundant.