A. A. Milne is chiefly known for his Winnie the Pooh stories, and his collections of nursery rhymes, When We Were Very Young, and Now We Are Six, all of which overshadowed his more serious plays and novels. One of those novels was a mystery.
Wikipedia tells me that The Red House Mystery was both popular and critically acclaimed. I'm not sure why. It's a competent but uninspiring locked room story, and if I could find any external evidence, I'd say it was written primarily to show Conan Doyle that Milne could do better.
Milne starts off with oblique references to Sherlock Holmes, but quickly moves to full-fledged, if light-hearted, mockery. Yet, while Milne jeers at Holmes' unique intelligence and retentive memory, his own amateur detective himself relies on a remarkably detailed eidetic for several issues. Milne works hard to address tired stereotypes of the detective genre, but his story is so carefully and obviously constructed as to take all the joy out of the story. Yet, despite all that work, his solution doesn't make a great deal of sense, and the clues aren't provided in advance. Often they come up through a close examination of the protagonist's eidetic memory - not something the reader has access to - but sometimes it's just crucial new information. For all his jokes about Holmes and Watson, reading the story feels very much like being led around by someone who's just having his fun with us. Even so, Milne leaves some fairly basic plot holes that we just have to ignore.
The book certainly shows another side of Milne, and I'll be trying his plays, but I can't say I'm sorry this was his only mystery. Point to Conan Doyle.