Blizzard of the Blue Moon by Mary Pope Osborne

The tree house sends Jack and Annie to New York City during the Great Depression, where they must rescue a unicorn trapped in a mysterious museum. With a powerful blizzard raging outside, they brave icy streets and magical challenges to break the unicorn’s spell. Their journey requires clever thinking and belief in the impossible. This tale weaves enchantment into real-world hardship, showing how hope can shine through even the coldest night. It’s a magical and emotional winter rescue.