The Case-Book of Sherlock Holmes

The Case-Book of Sherlock Holmes appeared in 1927, three years before Doyle's death, and it gathered the last twelve Holmes stories he would ever write. By this point Doyle had been writing Holmes off and on for forty years, and the late-career strain shows. The stories are uneven. Doyle himself wrote a preface admitting he was hoping readers would forgive any decline in quality, which is not the most confident way to launch a final collection.

That said, some of the late stories are genuinely strange and interesting. The Adventure of the Lion's Mane has Holmes narrating in his own voice, which never happens elsewhere in the canon, investigating a beach death. The Sussex Vampire is a domestic puzzle with a pleasingly mundane explanation. The Three Garridebs is famous mostly for Watson getting shot and Holmes's brief panicked response, which is the closest the books ever come to overt emotion between them. The Mazarin Stone is structurally odd because Doyle adapted it from his own one-act play. If you have read the rest of the canon, this is the closing chapter, warts and all.