It takes a special person to carry off this kind of illustration, which harks to James Marshall. I'll have to look at more of Dormer's books, because it was pretty good--I'll need more convincing.
As far as the story goes, I was more impressed. The pen wants to be free to do what it wants and does what it needs to to be set free. A wee bit schmarmy, with the Rousseauian view of the child, but this fits well with a child-centered approach to child characters.