The initial copyright is 1958. When a book is still being published, recommended, and used as required reading again and again across various curricula. Our original copy was a hand-me-down Weekly Reader hardcover from my Enginerd's childhood. Then we found another hardcover copy, well-loved and beaten up at a used book store. Now, thanks to the I Can Read! subscription, my son has his own pristine paperback copy with vivid color illustrations.
Note, there is also an animated short telling this story on some streaming services.