I have used the color index countless times - it saves me time and frustration when I'm trying to pick out the right combo. Love the CMYK & RGB formulas and how the book is organized. Idea index is wonderful for logo ideas and passing up a creative slump. Layout index - I've read cover to cover but not in the order the layouts were presented in. I don't want to copy the layouts they provide as examples so usually I give the book a quick glimpse when I'm stuck. For all three books - most times a glimpse is all you need to get the juices flowing.
This set has a lot of great examples, and helped me see some ideas for assignments and projects that I ended up using. My rating indicates that I've finished the books, but they're not really something to sit down and read cover to cover, more likely one would keep referring back to them as the need arises. At least that's how I've used them so far.