You will need to find 3 secret messages / enigmas of kirwall / special codexes each year in kirkwall. There are 3 years, which means there is a total of 9 messages. Actually, there are 4 'enigmas of kirkwall' every year. I show you the 3 easiest ones to get the trophy or achievement fast and easy.
The ones in Act 1 and 2 can be collected at any time. In Act 3 you have to do certain quests (On The Loose, Justice, The Last Straw) to reach the enigmas.
It's important to know where each Act begins and ends! Everything before the Deep Roads Expedition is Act 1 and everything after killing Arishock is Act 3.
Warning: The 3rd codex is invisible for some reason! Simply go to the shown picture (the codex is inside the right one) and keep pressing the button you're always using to pick something up (depending on your console).