Just a headcanon

I don't know why but I always like to think that John Uskglass created the King's letters when he was a boy (they could hardly be considered the King's letters back then). That is because creating your own letters sound like something a child will do. And I can imagine a very cute John Uskglass (everybody is cute at some point, including the Raven King!) being enthusiastic about this childish hobby. Maybe he had heard about writing and reading from Thomas (who has lived long enough among humans to know about the swirly thingies that spoke what you thought) or from other human slaves. Then he might have been overcome with the need to do the thing humans did but in his own way.

I also like to imagine that Thomas realized this was a childish thing to do but he also liked seeing John Uskglass having fun and maybe he might have even took some part in it by helping John creating his own letters. They might even use the letters to secretly communicate with each other.

Or this is all just my need to imagine John Uskglass as a cute boy before he turned emo (although he could be cute and emo at the same time for all I know).