There are a total of 5 "The islanders Almanac" magazine issues in the Fallout 4 Far Harbor DLC. You need to find all of them for the "The Islander's Almanac" trophy / achievement. Besides that, they also unlock perks and recipes unique to the Far Harbor DLC. You can track which ones you have, and don't have, by opening up the "Misc" section in the Pip-Boy and then scrolling down until you find "islander's Almanac" issues. Each one has a name, I have included the names below in the video timeline. None of them are missable, and you can get them at any time, even after finishing all of the Far Harbor quests.
Timeline:
[0:05] Issue #1 - Far Harbor's Sightseer Guide
[0:48] Issue #2 - Children of the Atom Expose'
[1:33] Issue #3 - Recipe Roundup
[2:06] Issue #4 - Pincer Dodge
[3:09] Issue #5 - Precision Hunting