In the comic book world, continuity can be a big deal. Does the newest story about Batman fit in with every story that’s ever been told about the character since the 1930s, by all the different comic book writers, plus radio, television and movies? Well, no.
So, then, the first thing you have to do is decide what’s canon and what isn’t. If you’re a Sherlock Holmes fan, you may like or dislike the various movies and TV shows, but the Doyle stories are the canon — nothing else is. Those other things are all just variations on the original theme.
But even when there’s only one writer, in only one medium, there can still be a lot of variety. Some writers just care more about continuity than others.
Tolkien created, as far as I know, a pretty coherent and consistent universe. Rex Stout, who wrote the Nero Wolfe mysteries, and who wrote each book from beginning to end, with no rewriting, quite often forgot the names of minor characters (and, heck, Doyle forgot Dr. Watson’s first name once). I would hate to meet somebody who ranked Tolkien higher than Stout or Doyle on that basis.
And then there’s Douglas Adams.
Each version of the Hitchhiker’s Guide to the Galaxy (radio, television, books, movie, stage) combined many familiar elements into new forms. By its nature, there cannot be a “canonical” version of the story. Which is fine, of course.
Adams wrote a script for Dr. Who once. It wasn’t produced, so he removed the Dr. Who elements and rewrote it as a novel, Dirk Gently’s Holistic Detective Agency. And then it turned out that the original Dr. Who script has now been produced as an audio drama by the wonderful folks at Big Finish, which I look forward to hearing.
I’ve been thinking about this, since I just wrote a scene for my current story that’s a variation on a pivotal scene from the novel U-town. That scene is shown three different times in the novel, but it’s always the same scene, just from different points of view. This time it’s actually a different scene (in fact from the point of view of a character who wasn’t even present the other time). So, it’s been interesting, and fun, to play a variation on a very familiar theme. Not something I usually get a chance to do.
Of course, most people avoid the whole question by writing each book as a standalone.
For some reason.