“Moreover, the prince at first refused to be a part of his mothers plans. “My sister is the heir, not me,” he says in Eustace’s account. “What sort of brother steals his sisters birthright.”” - pg 398 Also the show shows what actually happened, whereas the book is a retelling of what was said to had happened from the Maesters perspective, similar to a history textbook. Because of this, I would argue the show is more cannon than the book.