And Tolkien's Letters from Father Christmas.
Everyone knows The Lord of the Rings and The Hobbit, and plenty know about The Silmarillion and the other books, even if they haven't actually read them. Then there is a little-known collection of letters that Tolkien wrote to his children during the 1920s and 30s, now held in Oxford's Bodleian Library. 
The letters come from Father Christmas, and in the 1995 edition that I own these are reproduced and mounted in the book with facsimiles of the envelopes.
Father Christmas's handwriting has an ornate but shaky character that must have taken ages to write (presumably Tolkien's attempt to disguise his own hand); the illustrations are beautifully assured. Occasionally the letters are written by others, such as Ilbereth the Elf on the occasion that Father Christmas was too angry and upset to write himself – the North Polar Bear had fallen asleep in the bath with both taps running and his hind paw on the overflow. This wasn't at all unexpected, it seems that the NPB was always doing something silly, like the time in 1926 when he was responsible for "the biggest bang in the world" by turning on another "tap", the one that released all the Northern Lights for two years in one go.
In his final letter, Father Christmas writes:
Dear children,
Now I shall have to say goodbye, more or less. I shall not forget you. We always keep the names of our old friends, and their letters; and later on we hope to come back when they are grown up and have houses of their own, and children…
Your loving Father Christmas
How I wish I'd kept my correspondence with Santa Claus.