The following is from the "Credits and Acknowledgments" page in the book.

Winnie-the-Pooh is the creation of A. A. Milne and E. H. Shepard, originally seen in When We Were Very Young (1924) and Winnie-the-Pooh (1926).

When We Were Very Young entered the public domain in the United States on January 1st, 2020.

Winnie-the-Pooh, as well as the characters Winnie-the-Pooh, Piglet, Eeyore, Owl, Rabbit, Kanga, Roo, and Christopher Robin, entered the public domain in the United States on January 1st, 2022.

Winnie-the-Pooh got his name from a bear named Winnipeg in the London Zoo, who the real life Christopher Robin visited in the 1920s. His original name was Edward Bear. This isn't important copyright information, but it’s a fun piece of history!

I thought that last bit was nifty, and I didn't know before today!