"A Detective New to Fiction"

Caveat: This post is only for readers who prefer seeing the first appearances of stories they like, especially those with illustrations (illos), including maps and diagrams, that seldom 
get reprinted in book publication.

Detective fiction aficionados (as well as Winnie the Pooh fans) should already be well acquainted with this particular book by its usual title, A. A. Milne's The Red House 
Mystery (1922; Wikipedia HERE), but a few might not know it was serialized as The Red House Murder. You can find the complete novel at Project Gutenberg (HERE).

THE RED HOUSE MURDER.
By A. A. Milne (1882-1956).
First appearance: Everybody's Magazine, August-December 1921.
Serial novel (5 installments, 103 pages).
Online at Hathi Trust.
(Note: Some of the text and illos are faded but still legible.)

Part I: 19 pages. Online (HERE). A three dimensional diagram of the Red House is on page 22 (HERE).
   "All of us have the Sleuth-urge, the desire to solve crime mysteries. Mr. Milne confesses to it and that's why we have this novel. He thought, as he says, of a good way to commit a murder and there followed this story of mystery, thrills, humor and a detective new to fiction."

Part II: 21 pages. Online (HERE). A plan of the Red House and grounds is on page 77 (HERE).
   "An Unusual Mystery Novel with a Detective New to Fiction. Was the Killing an Accident and Did Mark Lose His Head and Run Away?"

Part III: 20 pages. Online (HERE). A diagram of the entrance to the secret passage is on page 75 (HERE).
   "While Cayley and the Inspector Drag the Pond, Antony, with Sinister Forebodings, Explores the Secret Passage."

Part IV: 24 pages. Online (HERE). Another plan of the Red House and its surroundings is on page 66 (HERE).
   "Antony and Bill Pursue Their Investigations and Arrive at One Definite Conclusion."

Part V: 19 pages. Online (HERE).
   "Antony Gillingham Does Some Very Shrewd Guessing and Solves the Red House Mystery."