It’s been 28 years since world-acclaimed children’s book author Theodor Seuss Geisel, or “Dr. Seuss,” passed away, leaving behind more than 50 beloved published works, from “The Cat in the Hat” to “Green Eggs and Ham.” Since 1995, six additional Seuss books have been completed and released posthumously by Random House and a seventh is now on its way.

“Horse Museum” is Dr. Seuss’ first nonfiction, non-rhyming children’s book and it’s set to hit bookstores across the U.S. on Sept. 3. There will be 250,000 copies released of the first printing.

“’Horse Museum’ is not a traditional story book,” said Susan Brandt, president of Dr. Seuss Enterprises, in an email interview. “It contemplates what art is all about and teaches children how to think about art. While it’s unique from Ted’s other books, Dr. Seuss’s ‘Horse Museum’ screams Dr. Seuss because Ted does not talk down to children in this book. He uses his unique and genius way of writing for children that respects and ignites children’s imaginations.”

In the fall of 2013, Brandt received a call from Dr. Seuss’ wife Audrey Geisel — who passed away just last year — and his assistant Claudia Prescott. The two had discovered, in a hidden closet behind a bookshelf at Seuss’ home in La Jolla, a box of the author’s old manuscripts. Inside the box was a folder marked “Noble Failures,” filled with uncategorized drawings and illustrations, a more complete project titled “The Pet Shop” — which was published in 2015 as “What Pet Should I Get?” — and lastly, a collection of sketches titled “Horse Museum.”

“I was so excited,” said Brandt. “It was indeed like discovering treasure!”

In Dr. Seuss’ latest picture book, a horse in a formal suit and bowtie takes a group of school children on a tour of a horse museum, filled with various depictions (sculptures, photos and paintings) of horses from famous artists such as Deborah Butterfield, Jacob Lawrence, Pablo Picasso, Jackson Pollock, and others. The book explores the history of art and distinctions between techniques; cubism versus realism and expressionism versus impressionism.

“I was surprised how much I learned about art just by doing the book,” said Andrew Joyner, an acclaimed children’s book illustrator based in Australia who was hired by Seuss Enterprises and Random House to illustrate Seuss’ new book. “I love the concept…Dr. Seuss’ idea that you can understand art by looking at how different artists have looked at horses, or painted horses. It just shows how endlessly creative he was.”

Though Joyner’s art illustrations differ from that of the well-known “Seuss style,” Joyner’s artwork still captures the childhood charm and lighthearted spirit of Seuss’ storytelling. But still, little Easter eggs have been scattered throughout the book, certain pages featuring beloved Seuss characters like the Grinch, Horton the Elephant, and the Cat in the Hat, all original illustrations by Dr. Seuss.

“I wanted the book to also feel like a bit of a tribute to him,” said Joyner. “I didn’t want to copy Dr. Seuss’ style because I don’t think I’m capable of that. But I wanted the spirit of his drawings in there. I think he’s an amazing artist and he’s had a huge impact on my childhood. So, the publishers and I thought this was a nice way to have this 60-or-so-page book be a homage to him.”

“We get to ensure every generation has the opportunity to experience these amazing characters and stories,” added Brandt. “I hope that children, and adults as well, will understand and embrace the core message of this book, that art is what an artist sees in something…This is a very sophisticated yet simple message that I hope children will embrace both when they see art and when they are inspired to create their own art.”

“Dr. Seuss’ Horse Museum” will be available for sale on Amazon.