Here is a 100% walkthrough of Chapter 8: The Letter including all the secret locations and the missable achievement
Obsessive.
Secret / Collectible #1 - 3:10
Secret / Collectible #2 - 5:39
Secret / Collectible #3 - 7:06
Secret / Collectible #4 - 7:57
Secret / Collectible #5 - 9:11
Obsessive - 0:18 to 9:50
The Obsessive achievement in Unravel worth 100 pointsBreak all the ice on the way to the letterbox in one go