Earlier I mentioned the idea of writing books consisting of multiple pieces which fit together but can also stand alone. Genre fiction often does this with episodes, by adding sequels and prequels to a story. George Lucas’s Star Wars was originally conceived as three sets of trilogies, even though the first film shot might also have been the last. It wasn’t, and so the second and third episodes of the middle trilogy were filmed. Then fifteen years later the first trilogy was filmed, to the regret of many.
There are more intricate possibilities for fitting together stories, though they aren’t explored often. When I was in graduate school there was a fair amount of buzz when PBS presented The Norman Conquests, a set of three plays by Alan Ayckbourn:
Each of the plays depicts the same six characters over the same weekend in a different part of a house. Table Manners is set in the dining room, Living Together in the living room, and Round and Round the Garden in the garden. Each play is self-contained, and they may be watched in any order. Some of the scenes overlap, and on several occasions a character’s exit from one play corresponds with an entrance in another.
Sounds gimmicky, but in practice it wasn’t. Instead the playwright managed to portray the multiple threads of the story from varying angles; each play covered much the same ground, but revealed new aspects of a story you already knew something about from the other plays.
More important, maybe, it scratched an itch that is usually there when reading a good story where minor characters are engaging—what exactly are those folks up to when they are offstage?
I don’t know exactly how you would take the three plays of The Norman Conquests and turn them into an accordion-like story where you could choose to follow different paths or not. But the raw materials are there.