Speculation! It's what the fandom loves most doesn't it? This seemed like minor news when Amazon listed it the other day, but it has exploded around the fandom as people wonder why Sunny isn't rocking the wings and horn she obtained at the end of the My Little Pony: A New Generation movie while retaining her new hair colors. We still don't know exactly how alicorn-mode works for her, and this seems to confirm that it's not a permanent change, but rather a temporary one.
Unfortunately Amazon doesn't have any specific info on the book, other than it's 24 page length and release date of February of next year. I guess for now, we can just keep an eye on upcoming 2022 merch and see if they keep rolling with this new hair coloration for Sunny.
For now, you can find the book over here.
Thanks to Olive, Dylan, Tryhard, and the many others that sent it for the heads up!