Mary Roberts Rinehart 
The Circular Staircase [EPUB ebook] 

Mary Roberts Rinehart was an American writer, often called the American Agatha Christie.Rinehart published her first mystery novel The Circular Staircase in 1908, which introduced the ‘had I but known’ narrative style. Rinehart is also considered[by whom?] the source of ‘the butler did it’ plot device in her novel The Door (1930), although the exact phrase does not appear in her work.