Many of the characters are named after detective authors:
Mary Roberts Rinehart ("The American Agatha Christie") wrote her first book The Circular Staircase in 1907.
Richard and Frances Lockridge collaborated on a series of mysteries starring "Mr. and Mrs. North" beginning in 1940.
Earl Derr Biggers is the creator of Charlie Chan.
The Gardners are named after Erle Stanley Gardner, creator of Perry Mason.
Mrs. Allingham is named after Margery Allingham, author of the Albert Campion stories.
The butler is Doyle for Sir Arthur Conan Doyle.