A Boy Called Christmas by Matt Haig

A young, earnest boy named Nikolas journeys to a snowy, magical North in search of his missing father and instead discovers a hidden land of elves, a frail but loyal reindeer, and a series of challenges that test his courage and kindness; through friendship, cleverness, and small acts of generosity he brings hope to a bleak world and helps shape the origin of the figure who will be known for bringing gifts to children.

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