As far as I can tell, the only way to miss anything is if you don't explore the whole map. All choices give different letters to read, the point is to read all that are available to you. Here are some tips to keep in mind while a solution to one of the big problems is located at the bottom of the guide.
Every map has a point of no return location. When you hit it, go back to your tent and scroll through all letters and notes.
Buy the watchtower upgrade as soon as you can, this can reveal points on the map that you would have missed. This is very important for the second tip.
Some of the hand icons on the map, the ones you might avoid because you think they are resources, are actually letters and notes within the game. You need to read these, especially when you return to Rivia.
In the Dwarven map, while exploring the location where
, there are towers with Dwarven notes on them. Read those.
When you return to Rivia, there are propaganda posters of you in the first half of the map, you need to find and read all of these before you get to the printing press. Choose your own propaganda posters and then read all of those that you find in the rest of the map. I am unsure if this is actually important for the achievement, but I did it anyways to be safe.
The most important thing that seems to get a lot of players is the
After the fight, there is one final letter available to you in your tent. A lot of people (including me) missed this on their first playthrough. If the achievement didn't pop and you think this is it, just reload the chapter and do it again. Takes all of 10 minutes to be sure instead of starting the whole game over.