Although this is listed as for four- to eight-year-olds, a nine-year-old would still find it worthwhile: Greek Gods and Goddesses (Dover Coloring Book). Here is my review of it from amazon.

This book is part of what I have elsewhere called a treasure trove of a coloring book series from Dover. I purchased it for my grandson (eight, going on nine), but I have many of the volumes in the series for myself. The illustrations are attractive and challenging. However, most important, the information conveyed about Greek mythology is more accurate than that found even in some of the most popular handbooks of Greek mythology (Edith Hamilton and her ilk). It is an admirable way to lead a child into the joys of the preeminent book of Greek mythology, Robert Graves, The Greek Myths.

One could also use Padraic Colum’s A Children’s Homer as a useful preparation for Graves’s book, which is hands down the single best book of Greek myths for all ages, although for a nine- to twelve-year-old, it may require parental assistance.