I read this book as a child in the 70’s. A brother and sister’s family moved to a new town. Their obnoxious and rude neighbor boy, I think his name was Jamie, played with them. They didn’t like him but they didn’t know anyone else so the put up with him. They discovered an oven in an old shed on their property. They baked a batch of cookies in it. Jamie selfishly wolfed them down and turned into some king of animal. The oven had magical powers if you ate something that was baked in it. That’s all I remember. I hope someone can help.  Thanks.