The 'wicked little letters story' might be a fable - like story where the letters themselves are personified. They could be 'wicked' in the sense that they play tricks on the characters in the story. For instance, the letters could rearrange themselves to change the meaning of important signs or messages, leading the main character astray. It could also be a story about a young child who discovers a set of letters that seem to have a mind of their own and create all sorts of mayhem in the child's otherwise ordinary life.
The 'wicked little letters story' could potentially be about a series of letters that cause trouble. Maybe the letters are part of a mysterious message that leads to some sort of chaos or they could be letters that are misused in a comical or even a somewhat devious way. For example, if it's a story set in a small town, perhaps the letters are used to spread false rumors or create misunderstandings among the townspeople.
I'm not sure exactly which 'wicked little letters story' you mean. It could be a very specific tale known in a certain community or a personal story. There might be some mischievous or naughty events involving letters in it, but without more context, it's hard to say more.