The Magic-Lantern Murders by Carter Dickson

On the eve of his wedding to Evelyn Cheyne, who he met and fell in love with during the course of The Unicorn Murders, former spy Kenwood Blake is summoned to Devon by spymaster Sir Henry Merrivale, and asked to take part in an undercover operation. Despite being assured he'll be back in London in time for the wedding the following morning, Blake is unconvinced.

Once in Torquay, he is sent to investigate Paul Hogenauer, a local scientist of German heritage, who has been dabbling in spiritualism and the occult a man Merrivale and Devon's Chief Constable Colonel Charters suspect of anti-British espionage. But, on entering his house, he finds Hogenauer dead in a chair in his sitting room, showing all the signs of strychnine poisoning.

When he finds Hogenauer's friend and confidant Dr. Keppel dead in his hotel room in Bristol, also apparently from strychnine poisoning, he has a seemingly impossible mystery to unravel. Can he solve the clues, bring the killer to justice and still get to the church in time to get married in the morning?