Moonshadow by Angela Carter, illustrated by Justin Todd (Victor Gollancz Ltd., 1982).
Apparently a rather rare book, this was one of my favorites as a kid. My dad and I found it at a used bookstore when I was little and I loved the way he would read me the zany story and we would find the hidden pictures within the illustrations.
The book is about a boy named Tom who likes to play with his shadow, casting shadow puppets on the wall before bed. One night, a goblin, who says he's the Man in the Moon, jumps out at Tom and tells him his shadow is tired of him playing games with him and wants a night of freedom where he doesn't have to follow him around all the time. Off his shadow goes, in the shape of a cat, and Tom must run to catch him. The illustrations feature many optical illusions and hidden pictures. In the beginning there is also a map for a board game that you can play with dice.
In researching the author and illustrator, I discovered that they are both relatively well known in Britain. Carter is known for her feminist and magical realism novels and Todd seems to be best known for illustrating British children's classics like Alice in Wonderland and The Wind in the Willows.
This is definitely a unique children's book! Maybe a little too scary for some young kids, but it was enchanting for me.