If there's one distinctive thing about the Discworld series--beyond Terry Pratchett's humor--it's that no matter which characters happen to be the main ones for a particular novel, the setting is the same. One flat world on the backs of four elephants, who are in turn standing on the shell of an enormous turtle called Great A'Tuin.
There is no particular reason why a majority or all of an author's books can't be set in the same world, over a variety of genres and/or age groups, with a collection of different characters. However, there are some features which make exploring the same place much more intriguing than if it had been left with only a single novel, that not all books have.
1. The existence of unexplored regions.
Rehashing the same places with different characters is interesting enough, but when they travel off someplace mentioned in previous stories that remained relatively unknown, then it really adds a new dimension.
Example: The Hobbit and The Lord of the Rings by J. R. R. Tolkien. The Fellowship followed a different path than Bilbo Baggins, encountered different creatures, and existed setting that may as well have been a separate world--except for the knowledge of locations linked to the previous story. This can also be done by setting a story in a different time. Example: The Blue Sword and The Hero and the Crown by Robin McKinley. A different landscape and people, many years apart, but the same continent.
2. Different cultures.
There must be a range of characters (people or other beings, as it may be) that, like the setting, are put into the background for some stories but brought out in others. Again, it's much more interesting when you see another side to something that was less well-known before.
Example: Fire and Graceling by Kristin Cashore. Fire is the companion novel to Graceling, set a little while before Graceling begins, on the other side of the mountains. A very different kind of magic and people, but with connections to each other culturally that only the reader notices.
3. The world must be big enough.
This relates closely to #1. If, say, you set your novel on a small set of islands, you are limiting yourself to that range. And while an archipelago can be excellent for a slave uprising (Example: Trickster's Queen by Tamora Pierce), there is a limit to how much you can pack onto it.
Same if you pick a setting that is too closely like good old real life--unless you happen to write Literary Fiction (which, for the record, I think is awesome) that doesn't leave much to the SF/F-style world building I'm talking about here. Once you've explored it once, then you must leave world building doors open if you want to go further. It's much like writing the second book in a series: if there isn't any opening, trying to continue it will just feel like a pointless extra (Example: City of Fallen Angels by Cassandra Clare).
How about you? Do you agree or disagree with the points I've made? What do you think are important elements of writing in the same world, for more than one book or series? Do you have a favorite cross-book world?
-----The Golden Eagle