Story related and cannot be missed.
This is for completing chapter 3, getting behind the bookcase by either:-
Prying it open with a crowbar OR
Figuring out the globe puzzle to have it unlock automatically. (For getting all of the achievements though, use the globe)
2 Achievements unlock!
Here is my 100% guide for this game and to see where and how to unlock this, go to 36:30 in playthrough 1 video.
Hope this helps