Dashiell Hammett wrote five novels. In 1975 Picador published a paperback collection of Hammett's work entitled, The Four Great Novels. The Thin Man was not one of them. Perhaps the editors did not consider it great because the tone is light. Comedy always gets less respect than tragedy. Otherwise, in The Thin Man, the crime is as weighty, the puzzle is as perplexing, and the solution is as brilliant as in the other four.
And The Thin Man has something else in common with perhaps the greatest of Hammett's novels. About one-third of the way through The Maltese Falcon, Sam Spade tells Brigid O'Shaughnessy about one of his past cases in which a Mrs. Flitcraft hired him to find her husband. The case has no apparent connection to the plot of the novel. Numerous critics have struggled to explain the presence of this story, which has come to be called the Flitcraft Parable. My favorite of these discussions is Jim Nelson's.
Similarly, In The Thin Man, also about one-third of the way through, Nick Charles gets out his copy of Celebrated Criminal Cases of America, and opens it to the entry on "Alfred G. Packer, the 'Maneater.'" He gives the book to Gilbert Wynant, a young man who wants to learn about investigating crimes, and who, for no apparent reason, has asked about cannibalism in the United States. Hammett inserts the full text of the article in the novel. It runs a bit longer than the Flitcraft Parable, about 1750 words.
So far I have not found any commentary about why Hammett included this seemingly irrelevant story in The Thin Man. If you are aware of any, please mention them in the comments below. At the very least, I think this article on Packer, should be considered alongside the Flitcraft Parable when trying to determine what Hammett was up to with these digressions.