Link to October 12 New York Times article, "Behind the 'Wimpy Kid' Phenomenon".
Excerpt: But given the books’ powerful appeal among both girls and boys, child development experts say parents have a lot to learn from Greg and company. While books like the Harry Potter series create an imaginative fantasy world, the Wimpy Kid books give us a rare glimpse into a child’s ethical mind.
“It really captures the struggle of a child that age trying to figure out what it means to be a person,” said Dr. Joshua Sparrow, a child and adolescent psychiatrist at Harvard Medical School. Dr. Sparrow read the first Wimpy Kid book after a young patient told him about it.