Former San Francisco 49ers quarterback Colin Kaepernick has written a book, "The Perilous Fight," which will be released on Sept. 15, 10 years after he first took a knee during the American national anthem to protest racial injustice and police brutality. The book is equal parts memoir and manifesto, tracing the off-field battles that transformed a single act of protest into a movement that changed American sports and culture forever. It is a story about identity, sacrifice, and the cost of courage.