Eric Carle, the author and illustrator of the timeless children’s book The Very Hungry Caterpillar, died May 23 at 91.

His death was announced Wednesday on his Twitter account and by his publisher, Penguin Kids.

“It is with heavy hearts that we share that Eric Carle, author & illustrator of The Very Hungry Caterpillar and many other beloved classics, passed away on May 23rd at the age of 91,” Penguin Kids tweeted. “Thank you for sharing your great talent with generations of young readers.”

Jonathan Wiggs/The Boston Globe via Getty Images Children’s book author and illustrator Eric Carle

Carle’s most famous book, first published in 1969, sold millions of copies across the globe, even if the author himself wasn’t originally sure it would be a hit.

“I didn’t think it was going to be an extraordinary book,” Carle told EW in 1994, as the book turned 25. “But children love it. Caterpillar reassures young kids that ‘you scrawny, ugly little thing will grow up and fly and display your talent and beauty.”‘