Taken At The Flood by Agatha Christie in Paperback

 

A man mysteriously comes back to life and a stranger's body is discovered in his room. Following his death in a bombing during the London blitz, Gordon Cloade leaves behind an attractive widow who inherits the entirety of the Cloade family fortune. However, Poirot is approached by the deceased's sister-in-law, who believes her late brother-in-law's first husband is still alive according to "spirits." Poirot is tasked with locating the missing person, but is unsure of the woman's true motives for seeking his assistance.