Joe and I were just talking about how some authors write characters from one book of theirs into others. The effect is that seemingly unrelated books form part of a cohesive world. For example, a major character in an early book makes a cameo in a later story or novel, or a minor character becomes a protagonist. We agreed that this phenomenon should be more common, but Joe could only think of two writers who do this: Elmore Leonard and David Mitchell, and all I came up with was Kurt Vonnegut. Are there any others you can think of? Types of lit that don't count: historical drama or fiction and books that are explicitly sequels or series.
In TV and movies, we have the spinoff, but I feel like there should also be a literary term for this!