This book retells the story of Cadmus as a vehicle to explain the development of letter forms.
Sound dull? It is a little. More, it is confusing. I'm familiar with the idea of etymology and the alphabets in question and had to add in that knowledge. I don't think this would be a good intro for a child because it doesn't give enough information while at the same time demanding a lot of thought.
This might be good for someone who was already beginning to teach a child Greek -- I think knowing the letters and how they're pronounced would make the book a lot easier to grasp. There is a comparative alphabet chart at the back; looking at that first might help.
Another suggestion I would make is to get your child another version of the story of Cadmus before reading this. I think trying to follow both the narrative and the scholarly part at the same time is pretty confusing on a first read.
I would recommend this primarily to parents who are homeschooling or giving their kids additional out-of-school lessons. Or, of course, for adults who like picture books. Rumford's illustrations are quite good if you enjoy ancient Greek art.
I didn't love this book, but I'm glad it exists.