John Dickson Carr was born in Uniontown, Pennsylvania, in 1906. It Walks by Night, his first published detective novel, featuring the Frenchman Henri Bencolin, was published in 1930. Apart from Dr Fell, whose first appearance was in Hag's Nook in 1933, Carr's other series detectives (published under the nom de plume of Carter Dickson) were the barrister Sir Henry Merrivale, who debuted in The Plague Court Murders (1934).
the book actually read & rating given are for The Unicorn Murders, written in 1935, to give credit to John Dickson Carr instead of putting under his pen name Carter Dickson; this is Sir Henry Merrivale #4