The Father Christmas Letters

Between 1920 and 1942, JRR Tolkien wrote a series of letters to his children purportedly from Father Christmas, in which he told them about his life at the North Pole and his helpers and acquaintances - most prominently the good-hearted but accident-prone North Polar Bear. The letters were illustrated by Tolkien himself in his idiosyncratic style.

With the public appetite for more Tolkien writings after his death in 1973, the Letters were collected and published in 1976. An expanded version with more material, re-titled Letters from Father Christmas, was released in 2004.

Tropes used in The Father Christmas Letters include:

All Myths Are True: Father Christmas refers to his own father, Grandfather Yule, who represents the pre-Christian pagan winter solstice festival. He also mentions his 'Green Brother', whom we learn little about, but some people have speculated is the symbol of the summer solstice.
Badass Santa: Father Christmas leads his elves into battle against the goblins in the later letters (he writes that the goblins' strength is proportional to the rise of strife due to World War Two)
Cypher Language: The North Polar Bear invents one based on goblins' cave drawings and sends his own letters to the children in it.
Everything's Worse with Bears: Inverted, although the NPB does manage to upset quite a lot of things by accident.
Let's Get Dangerous: The North Polar Bear when he fights the goblins, being usually rather hapless and accident-prone.
Only Known by Their Nickname: The North Polar Bear's real name is Karhu, but this rarely comes up.
Our Elves Are Better: Unlike Tolkien's usual Elves, these are more the traditional Victorian version, gnome-sized. This doesn't stop them kicking arse.
Our Goblins Are Wickeder: The goblins here are also very small, essentially the elves' Evil Counterpart.
Really Seven Hundred Years Old: Father Christmas says he is as old as Christmas itself, giving his age as the year (around 1,930).
Shout-Out: One of the letters (written when Tolkien had started writing The Hobbit) contains an image showing both Smaug (as a cave painting) and Gollum (peeking round the corner of a cave).
- Father Christmas's elf secretary, who often adds notes in the margins of the letters, is named Ilbereth -- which should amuse anyone who's read The Lord of the Rings.
The Stars Are Going Out: A mild example, when the North Polar Bear breaks the North Pole (literally), the Pole Star turns red until it is repaired.
Theme Naming: The polar bears have Finnish names, either because Finland is appropriately northerly or just because Tolkien liked the language.