Sir Arthur Conan Doyle peppered his tales of Sherlock Holmes with allusions to other, untold stories, with the implication that he might tell them later on. probably the most famous such reference was in "The Adventure of the Sussex Vampire," in which Holmes explains to Watson a reference in a letter from a client:

     "Matilda Briggs was not the name of a young woman, Watson," said Holmes in a reminiscent voice. "It was a ship which is associated with the giant rat of Sumatra, a story for which the world is not yet prepared.

     Many writers have attempted to flesh out these two sentences, none terribly successfully to my mind at being Holmesian and/or "a story for which the world is not yet prepared."
     What's most infuriating to me is that most cannot even stick to the facts contained in those few words, i.e.:
     ~ It involves a ship named Matilda Briggs.
     ~ It involves something that could be referred to as the giant rat of Sumatra.
     ~ It is a story of which Watson was unaware.
     ~ It is a story for which the world is not yet prepared.
     Most attempts at the story have no problem with the first two, but many ignore the latter two. I can understand that they want the interplay between Holmes and Watson, but to ignore the last point is to ignore the very reason we are tantalized by the case. WHY is it a case for which the world is not yet prepared? What shocking secret or grotesque detail makes it so un-tellable?
     If it is a case of which Watson was unaware, some explanation for this must be given. There are two main periods of Holmes professional life that did not include Watson: before they met in "A Study in Scarlet," and during the years when Holmes was believed dead. These seem the most likely times, but a writer could easily find some other times, such as during Watson's marriage(s). To ignore it and insert Watson in the story seems lazy.
     "The Giant Rat of Sumatra" by Richard L. Boyer (Titan), recently reprinted in "The Further Adventures of Sherlock Holmes" series, ignores the second two points, which is bad, but (despite glowing reviews on Amazon) it also is poorly written and conceived. It is essentially a re-write/sequel to "The Hound of the Baskervilles," incorporating some of the same elements, such as Holmes seemingly sending Watson on ahead while secretly watching him and events; a mysterious animal trained to kill someone for revenge; and (SPOILER ALERT!) the same villain.
    There are other novel attempts and many  short story versions ~ including one in Ted Riccardi's "The Oriental Casebook of Sherlock Holmes" and Paula Volsky in Marvin Kaye's anthology "Resurrected Holmes" ~ but even the best fall short of creating the sense of mystery Doyle created in two short sentences.
     My favorite "version" is still the Firesign Theatre's comedy album "The Tale of the Giant Rat of Sumatra," a Sherlock Holmes parody that begins with Hemlock Stones starting to tell Flotsam the story, only to be interrupted with a new case that includes a lot of ratty references, including a character named Willard (after the then recent film), who in the end is revealed to be a "Suma-ratran." The jokes and references fly at a fast rate, and some find the humor induces more cringes than laughs, but for the brief period in which Holmes talks about the giant rat, he sticks closer to Doyle's words than many of the other chroniclers. It's available on CD through http://laughstore.stores.yahoo.net/.