Lestrade and the Mirror of Murder — M.J. Trow

Superintendent Lestrade and his team at the Yard are facing a series of murders, at first apparently unconnected. An ex-Merchant Navy man and his three nieces are killed when the harness of their trap breaks on a downhill gradient. An accident, surely? But what is the significance of the broken mirror in the Captain's pocket?
Not a stone's throw from the Yard, a female student of History breaks her neck at King's College. Misadventure, of course? The broken mirror in her belongings says otherwise. And out on windswept Beachy Head, a member of the Spanish aristocracy is found dead in a ruined lighthouse, with a broken mirror (not usually part of a cavalry officer's kit) lying beside him.
The clues accumulate, but so do the mirrors, and the murders... and the suspects.
'Mirror, mirror, on the wall, who's the guiltiest of them all?'
'Sergeant Cribb - look to your laurels.' - Portsmouth News