The writer of the "Baby Reindeer" book was Santa Claus himself. Santa Claus is the legendary figure who brings presents to children on Christmas Eve, and he is also known for his kindness and generosity. The story of the baby reindeer is a tale of friendship, compassion, and the power of love. Santa Claus wrote the story to teach children about the importance of caring for others and to remind them of the joy and magic of the holiday season.