There are a lot of editions of various combinations of the Professor Challenger stories, but this 1990 edition from Chronicle Books collects all the ones that weren't in the Penguin Classics version of The Lost World and Other Thrilling Tales, which is exactly what I needed. (As far as I can tell, there are no scholarly editions of the later Challenger stories.) I read them in publication order, not the random order they're printed in here, so that's the order I'll tackle them in.

The Land of Mist is the last Challenger novel. Amusingly, it begins with a disavowal of all previous Challenger stories (or maybe just The Poison Belt: "The great Professor Challenger has been-- very improperly and imperfectly-- used in fiction. A daring author placed him in impossible and romantic situations in order to see how he would react to them." Challenger brought a libel action against the perpetrator, but exactly who that perpetrator might be doesn't make a whole lot of sense, as both previous Challenger tales were supposedly written by the reporter Edward Malone (Land of Mist is in the third person), who here is on sufficiently good terms with Challenger as to be marrying his daughter!

But all that's sort of to the side, as The Land of Mist is just a terrible story. Written ten years after The Poison Belt-- an interregnum in which the Great War transpired-- the book is largely driven by Doyle's spiritualist beliefs, and it's less about Challenger than the most tediously dull and sanctimonious spiritualists you ever met. There might be some others who are fakes, but these ones, honest guv, they're the real deal. You can tell this because they're poor and virtuous. This goes on for almost 200 pages, and Doyle even includes an appendix citing his sources because it's all true. The only thing worse than plunging Challenger into this mess would be using Sherlock Holmes, so I guess we can be thankful that Doyle still had some sense and never went that far.

The other two tales are short stories, "When the World Screamed" and "The Disintegration Machine." (One of them mentions that Mrs. Challenger is alive, and she was dead in The Land of Mist, so I think Doyle retconned his retcon!) These are both pretty dull sf stories Isaac Asimov would call Stage Two, technology dominant; they'd fit right into an American sf magazine from the Golden Age, in that both focus on explicating some kind of technological idea (the Earth screams when you drill into it, you can disintegrate and reintegrate people) without actually telling a story around it or doing anything interesting at all.

As much as I liked The Lost World, the later Challenger stories published in this volume weren't nearly as good, alas. While "The Disintegration Machine" was quite enjoyable, I didn't find the "The Land of Mist" particularly interesting, given over as it is to Conan Doyle's forays into spiritualism and the occult. ( )