I have a fondness for detective stories that start out with utterly mundane mysteries (Charles Stross' The Concrete Jungle is perhaps my favourite iteration of the trope). This story has a wax museum curator coming to Sherlock Holmes to tell him that, of all things, the playing cards in the hands of the 'wax figures playing poker' diorama seem to keep changing.
It's an enjoyable little mystery, but what makes this memorable is that Holmes starts the story with a sprained ankle and confined to bed. When a case arrives, he mentions his brother Mycroft, and quotes a line from 'The Adventure of the Greek Interpreter': "If the art of the detective began and ended in reasoning from an arm-chair, my brother would be the greatest criminal agent that ever lived." Now that Holmes is himself confined to bed, Watson sees this as an excellent time for Holmes to solve the case and prove his superiority over his brother. Unfortunately this situation is never followed up at the end of the story, but even so it's a nice look at the relationship between Sherlock and Mycroft. And given that the son of Arthur Conan Doyle is co-writing it, it has a greater canonicity claim than most others.
Where to find it: The most recent printing of The Exploits of Sherlock Holmes was in 1999, and secondhand copies seem to be plentiful.