Wait till you do some of the quests in the Thalmor area. One of my characters has been jumped on twice by 3-men Thalmor assassination teams in the past two days. it's easier to kill the single DB assassin. Oh, and when you kill the Thalmor assassins, you find an interesting note on one of their bodies explaining the why, etc., and a warning to be careful because your character knows how to take care of itself. And she did, both times lol. Made some high-ranking people mad doing one chain quest there, I guess.
Lots of fun.
PS: The only thing that is rather odd, I think, is that you are being hunted by the DB assassins, and they have a note signed by Astrid (sp??) saying not to screw it up; yet you have no problems joining the DB with an active kill contract on your character.
are you talking about the main quest where you crash the thalmor ambassadors party while searching for info on the dragons? cause just recently did that and had none of those dirty little elves hunt me down (not yet anyway). I also know that that after that main quest random dialog will come out about that quest.
as for the letters, have no idea cause the main thing in the letters says that something along the lines of "wanting the dragonborn's power grow" so could be anyone. Someone with the empire, caus eof the septium blood in you, or someone with the stormcloaks cause the dragonborn comes from Nordic ancestors. Since the greybeards already give you out similar quest it could be them, or that dragon at the throat of the world Paarthurnaux trying to help you( or however you spell it ), other ancient dragons that appose aludin's conquest (there was more than one), Bosomer not getting along with the Altimer/Aldmier dominion.
Personally I think it could be the Nerevarine (Hero of elder scrolls 3 Morrowind) coming back from Akavir trying to recruit the dragonborn to retake Morrowind from the Argonian's. Or thats at the top of my list anyway, would make for an awsome expansion anyway.