Many of the crayons in Duncan's box are fed up. They've got a gripe, and they've written him a letter to express their feelings, and possibly get some retribution. 
 On each left-hand side page is the note from the crayon, and each right-hand side page boasts an illustration of that crayon's color. I love how the purple crayon admonishes Duncan for coloring outside the lines, then signs his letter "your very neat friend, Purple Crayon." 
 So very cute, you'll grin throughout the book.