The Horn Book includes an article called Hey, Al and the Choice by Kathleen T. Horning. Hey, Al, illustrated by Richard Egielski and written by Arthur Yorinks, won the Caldecott in 1987, even though it is, according to Horning, "clearly an adult's fantasy."
The entire article deals with the issue of Hey, Al being signaled out for an award for children's books when its protagonist is an adult. Horning says, "...I'm not sure it's a completely satisfying story for children. Essentially, it's a retelling of their mentor's masterpiece, Where the Wild Things Are, told from the perspective of a middle-aged man."
It's not a definitive article on picture books for adults, in general. Think of it more as a variation.