Not a book error. It's because M79.60 is not a complete code, it's a subcategory heading. You should see a symbol in your book, right in front of the code heading, that indicates a 6th character is required for a valid code.
The index is driving you to the subcategory heading, M79.60 Pain in Limb, Unspecified. Under the heading are listed all of the codes, with 6th character, by body part and laterality. You would search down and find the one most applicable for your case.
So pain in leg, unspecified or NOS, would be either M79.604 for right leg, M79.605 Left leg, or M79.606 for leg, unspecified.