I liked it...
But it's taking all my willpower not to give you zero because of the amount of rage you've just given me.
So, I got to the last screen with 11/12 letters, figuring if this was the end then there would be some way back to get the last one.
There isn't. I'm trapped. After playing this game for half an hour, there's no way back to find the last letter. So there's no way of finding out if I missed a secret ending.
If I'm wrong, please tell me.
If I'm right, then screw you. Let this comment serve as a warning to fellow completionists.