The Moving Finger by Agatha Christie

All is not well in the village of Lymstock. The village’s quaint country charm has been disturbed by series of virulent poison-pen letters. So far the letters have been dismissed as the harmless ravings of a frustrated and disturbed person, after all the accusations they have made are simply not true. But even are arrow shot blindly may hit its mark and the business of these letters becomes serious indeed.

With the police baffled there is only one person who can get to the bottom of this mystery. A person intimately familiar with the darkness that lurks within the human soul and the depths of depravity to which her fellow man can sink: Miss Jane Marple.