"One night, after thinking it over for some time, Harold decided to go for a walk in the moonlight
Children's picture book about a nursery-aged boy who climbs out of his crib at night and has an adventure in a world he creates with nothing but his purple crayon. He does this by drawing everything he "encounters", on the wall of his bedroom, no doubt. Harold interacts with his drawings as though they were real things, although they are only line drawings.
A simple yet enduring format, there have been several other Harold books, including:
Harold's Fairy Tale : Further Adventures With the Purple Crayon
Harold's Trip to the Sky
Harold at the North Pole : A Christmas Journey with the Purple Crayon
A Picture for Harold's Room
Written and illustrated by Crockett Johnson.