Black Coffee by Agatha Christie

Sir Claud Amory’s revolutionary new formula for a powerful explosive is stolen. Locking his house guests in the library, Sir Claud switches off the lights to allow the thief to replace the formula, no questions asked. When the lights come on, he is dead, and Hercule Poirot and Captain Hastings have to unravel a tangle of family feuds, old flames and suspicious foreigners to find the killer and prevent a global catastrophe.