when this mission is highlighted it says 'read the letter' ive read all 3 letters i have, burned papers, scrawled notes and spys notes but its not triggering the mission. am i missing something?

I had this happen too. I know it triggered when I picked up a note at one of the northern bandit camps. My guess is that it overlaps with the treasure hunt triggered at the bridge north of town and simply doesn't fire properly if you've already completed that.

just watched a youtube video, in the video the scrawled notes letter was in the quest items tab in my game its in the usable items tab, maybe this is the problem?

Same here, i looted final treasure before i got the papers and now this quest is broken. :(