Grace’s answer to “Is this actually about a white horse or is it another falsely advertised book with a horse on it?” > Likes and Comments

Could you kindly explain the metaphorical meaning please? Although I fell in love with this book, I struggle to understand the meaning of the horse/unicorn. I was confused that the book had the horse in the title, when the horse is barely mentioned in all the book. Perhaps he represents her purity of heart?