|The Hound of the Baskervilles|
But the conventional wisdom is nonsense.
Many of Arthur Conan Doyle's most memorable Holmes stories where written in the twentieth century, including The Hound of the Baskervilles. I could list many other examples of top-quality work in the last five books of the Canon. Even ACD's last two tales, "The Adventure of the Retired Colourman" and "The Adventure of Shoscombe Old Place," are excellent detective stories.
So why the disrespect for the later stories? Conan Doyle himself had a wonderful explanation in an essay called "Mr. Sherlock Holmes to His Readers," published in The Strand in March 1927. The essay later became the preface to The Case-Book of Sherlock Holmes, but without the following insightful paragraph:
"There has been some debate as to whether the Adventures of Sherlock Holmes, or the narrative power of Watson, declined with the passage of the years. When the same string is still harped upon, however cunningly one may vary the melody, there is still the danger of monotony. The mind of the reader is less fresh and responsive, which may unjustly prejudice him against the writer. To compare great things to small, Scott in his autobiographical notes has remarked that each of Voltaire's later pamphlets was declared to be a declension from the last one, and yet when the collected works were assembled they were found to be among the most brilliant.Scott also was depreciated by critics for some of his most solid work. Therefore, with such illustrious examples before one, let me preserve the hope that he who in days to come may read my series backwards will not find that his impressions are very different from those of his neighbour who reads them forwards."
Whether his explanation of the general preference for the earlier work is right or not, I agree with ACD's defense of his later stories. Do you?