Letters can be brilliant clues - whether they are used to sucker the characters in to your carefully-honed plot, or used as clues when the poor dears begin to flounder and don't know what to do next.
Here, in succinct one or two sentences, are described some 26 different letters. You will need to flesh them out a bit, to suit your plot or the characters themselves, but this provides a starting point for you to use such missives as potent plot-drivers. They can also make nice props to hand out, especially if you can manage a bit of calligraphy - and if that's beyond you there are plenty of interesting fonts to download for your word processing or art package to make them look good even if you, like me, do not write very well on paper!
I quite like #20: "A letter announcing that you’re next in the line of succession for a royal line beset by assassins." You could get a whole campaign out of that one, never mind an adventure or two!
At least, this wasn't what I first thought when I saw the product. 26 Letters? Oh, that would be the ones A through Z! I do not understand #1: the other 25 however give me plenty to be getting on with.