After much research I conclude that perhaps there is no definitive answer. Nobody can say for sure who it might be. Perhaps it's one of those non-canon things, allowing you to decide who is sending these letters. Some ideas I've heard make sense for some character types but make no sense for others. If I start by telling you my best candidates, and why, then the bickering and snarling can begin! So, as a do-gooder Nord warrior, Davimiir likes to ponder the possibility that perhaps, just maybe, he is guided by Ysmir Wulfharth-The Underking. He works behind the scenes after all. Of course, if I were a High Elf and a member of some shady organisation this would make no sense at all! And now for my 'clever dick' bit. Whilst wondering the great province of Skyrim one finds many books of course. There is a book called Dragon Language:Myth No More by Hela Thrice-Versed. Another candidate who possibly knows where all the the Word Walls are, I thought! I won't go into all the candidates otherwise I'll be here all week! Who do you think is sending you the letters?