The two books that I have been privileged to read this time are volumes 3 & 4 in the series. Each book is aimed at children 4-10 years of age. The younger child would enjoy the simplified pictures while the older child could read the words.
This is a not so subtle way for children to learn about a number but also to learn about the things and places around them and in the world. Some things in book three could be a little harder to grasp for the young child like the idea of living in a three dimensional world but that does not distract from the book as a whole. In fact, I believe it helps a child to imagine and for the older child a prompt to explore more.