Books: The Moonstone by Wilkie Collins

The Moonstone was written in 1868 and - as Wikipedia points out - is generally considered the first detective novel in the English language:

It contains a number of ideas which became common tropes of the genre: a large number of suspects, red herrings, an English country house, investigation by talented amateurs, and two police officers who represent the 'local bungler' and the skilled, professional, Scotland Yard detective.

You can read it online or download it for free by clicking here.