This achievement's progress is updated when you pick up each part of the letter. There's no need to actually read them, so you can skip their cutscenes. Also you don't really need to use the robe I used to create these screenshots, nothing here is really a secret with clues that the Detective robe can find.

The postwizard
— Read the entire letter from Meach in Dungeons & Gargoyles.

The first part of the letter is located right at the "Lovely Street 6", the first area you enter (not where the DLC starts). There are two postboxes there, both are in the sidewalk and are really hard to miss. One of them contains the Crowbar and the other one has what we need: the 1st part of the letter.

Destroy the second postbox in the area and pick up the letter.

The second part and the main reason this guide was created is in the area after Lovely Street 6. It's located behind the first house of the map, in front of which you encounter a zombie eating a dead knight (the one who drops a gun). Open the gate, kill the zombie if needed, go behind the house and if you hover your mouse over those sewer metal bars in the wall, you can see there's something that can be picked up, but you just can't really do it neither see what it is.

Simply attack the grate with your spell of choice: the second part of the letter will be sitting inside.

The third part of the letter you'll find in the second to last area of the DLC. That's the place right after the park. Right at the beginning of it you'll encounter the last postbox.

Destroy it, pick up the letter and breathe in that deep lore.

If you get to the town square (the part with the cutscene that ends with slimes spawning) and didn't get the achievement, you went too far.

There we go. If this guide has helped you out, don't forget to rate it. And if there is any way that I can make it clearer, do comment.