Before starting the game I searched high and low for a trustworthy guide for this achievement, but couldn't find one, so I gathered all the information I could find and managed to unlock it on my first playthrough, these are my findings:
(I was playing on the hardest difficulty, but I don't think it matters)
First of all, I got the achievement upon receveing a report during the epilogue, after a battle.
I'm not sure what actually qualifies as "spymaster letters", but reports also count, since I got the achievement after receveing one.
So, for the achievent:
* Do everything in every map, every question mark, every puzzle, every item, every golden chest and so on. Many of the battles lead to you receveing reports/letters afterwards, and many items also have letters within them.
To make this easier, be sure to buy the watchtower upgrade and send scouts (-50g) to reveal interest points on the map.
* Always read everything upon receiving, just to be sure.
* Choices don't matter. As far as I could tell there are missable letters/reports, I missed one in Mahakam
, apparentely the game only counts the ones you would be able to obtain, given your choices.
As long as you resolve everything on your map before finishing each chapter, it should be fine.
That's basically what I did in my first playthrough and managed to unlock the achievement without much effort.