The Curse of Mont Saint Cloud

Book 9 in a series of chronological stand-alone plots. January 1900, a double mystery in the English Channel. Dr Watson and the Countess are sailing across to France aboard an opium clipper when a storm forces everyone ashore at Mont Saint Cloud. The small island's reclusive owner - a professor dedicated to deciphering a mysterious codex (inspired by the Voynich manuscript) - is found dead, under mysterious circumstances.
Our sleuths suspect some dark secrets amongst the passengers. The voyage continues to Brittany, where someone is killing off Inspector de Guises's scandalous family - a rogues' gallery of misfits and criminals. The notorious clan has gathered at the family chateau for yet another funeral, and the inspector invites Watson and the Countess to help make sense of the murders - but they are spoiled for choice when everyone turns out to be a compulsive liar, swindler and thief.