The letter above is in Dipper and Mabel's guide to mystery and nonstop fun.

This'll be a really short one.
So if you take a look, all the letters on the left hand side are capitalized even if it's not the beginning of a sentence. And when you take the letters it spells out 'HELP ME ESCAPE OR BE DESTROYED.'
That may have been either a reference to the next episode or to Stanchurian Candidate. I'm leaning towards the Last Mabelcorn.

Thanks for reading. Please comment your thoughts, it would help.