Illustration by Eleanor shakespeare

It’s hard to imagine Agatha Christie coming back as a ghost (supernatural interventions, in her novels, prove to be red herrings) but if she did, she’d be pleased: 50 years after her death, brand Christie is booming. Her books sold more than 200,000 copies in the UK last year. A Christmas staple as reliable as Quality Street, she is a nostalgic shorthand for a lost world of luxury train travel and afternoon tea at the vicarage. Spin-offs abound: a biopic is in the pipeline, plus a BBC series, Hercule (seemingly based on the courageous premise: what if Poirot was young and hot?). A British Library exhibition now invites visitors to peer reverently at relics like a chemistry textbook once owned by the “Queen of Crime”.

Except, there wasn’t one crime queen: there were five.…