Look in any bookshop or library – chances are, you’ll find more shelves devoted to crime than to any other genre. Today there are scores of bestselling crime novels, with countless subgenres, but 150 years ago it was a new phenomenon. For TS Eliot, The Moonstone was ‘the first, the longest, and the best of modern English detective novels…a genre invented by Collins’. Dorothy L Sayers described it as ‘probably the very finest detective story ever written’.