Frederick Warne & Co., an imprint of Penguin Young Readers, will publish a new Beatrix Potter tale concurrently in the U.S. and U.K. this September. The Tale of Kitty-in-Boots was re-discovered by PRH Children’s U.K. publisher Jo Hanks, via a reference to the unedited story in an out-of-print Potter biography. Hanks made a trip to the archives at London’s Victoria and Albert Museum, and found three manuscripts and some sketches.

Kitty-in-Boots “is the best of Beatrix Potter,” Hanks said in a release. “It has double identities, colorful villains and a number of favorite characters from other tales (including Mr. Tod, Mrs. Tiggy-Winkle, Ribby and Tabitha Twitchit). And, most excitingly, our treasured, mischievous Peter Rabbit makes an appearance – albeit older, slower and portlier!”

The book was found handwritten in children’s school notebooks, with one rough color sketch of Kitty-in-Boots and a dummy book with some pencil sketches. Letters from Potter, also in the archive, revealed that interruptions, namely WWI, marriage, and illness, had kept her from completing the project.

The new book will be illustrated by Hans Christian Andersen Medalist and former British Children’s Laureate Quentin Blake. While the book was re-discovered two years ago, Hanks waited until 2016 to publish it, to coincide with what would have been Potter’s 150th birthday. The book will officially release on September 16.