The Crimson Cryptogram by Fergus Hume audiobook.
Young Dr Ellis, a struggling new physician, is enjoying a quiet evening smoking and enjoying conversation with his journalist friend Cass, when their mysterious neighbour, Mrs Moxton, bursts in upon them with startling news - her husband has been murdered! Rushing to the scene, the two men discover Mr Moxton, stabbed in the back. They investigate the body thoroughly, but find no real clues to his assailant except for a mysterious series of markings, scrawled in blood on the dead man's sleeve