Prose? By Dr. Seuss? Yep. And all the illustrations are black and white pencil sketches, with the exception of Bartholomew Cubbins' 500 hats, which stand out in bright red. It's an appropriate color scheme, seeing as Bartholomew's hat is the main protagonist in the story.
Two picture books into his illustrious career, it's already obvious that Dr. Seuss has a soft spot for the least powerful beings in a given society, and a healthy disdain for those who would rule over them. The 500 Hats of Bartholomew Cubbins
may not approach the near-revolutionary genius of [b:Yertle the Turtle|13133670|Yertle the Turtle|Dr. Seuss|http://d202m5krfqbpi5.cloudfront.net/books/1322708693s/13133670.jpg|18310150], but it sure gives it a solid go. A royal family almost undone by a hat with an etiquette-schmetiquette mentality? A young duke willing to behead our hero Cubbins to get the hat off his head? (I kept wondering if another head would pop up on Cubbins' shoulders as soon as the executioner's blade struck.) A king so shallow that he's willing to forgive the breach of royal manners when the replacement hats become more and more beautiful and spendy? No doubt, Dr. Seuss has little use for royalty.
As much as I liked the story for its politic, our toddler Sigourney disliked the unrhymed wordiness. Granted, she's still 2 weeks shy of her 3rd birthday, but I agree that the book's too long. Seuss could have lost a word or thousand and gotten the point across effectively. I recommend the read, but bring a lozenge if you're sharing the story with little ones.