The book has fold out pages which have actual samples of DMC floss in every color that the company makes, along with the reference number next to it. I use the book for every single project to satisfy the nagging doubt, "Is this color floss the closet match to the fabric?"
I spread the book out in good lighting and lay my fabric next to the floss samples to find the best match. About 50% of the time I discover that I do not have the needed colors on hand and I then have a good excuse to make another trip to the fabric store!
You may notice that there is not a blue floss in the picture. That is because I discovered that there is not a DMC shade of blue close enough to suit me. Having the book on hand has also encouraged me to use a wider range of colors with my embroidery instead of using the same favorite ones over and over!