For example lets say you are reading and really loving a book series by an author and awaiting the next book in this series. only instead of the next book continuing the story, its actually a prequel detailing events that happened prior to the main plot of the series. Maybe it fleshes out certain important characters or historical events in the series.
How would you feel about that?

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Depends, its bassically a flashback worth its own book. It can work well but it can't be too big before a finished work.
But if its just its own thing in the same world before the main story than please no. Let me finish my story first.
Robert Jordan wrote New Spring after book 10 before book 11. It was a fully fleshed novel (just shorter than his novels). On my rereads, I always start with that, because it helps the series seem more complete.
A bit annoyed because i would have wanted to read the prequel first.
If the content fleshes out the story then I love that. But don't give away the mystery
I'd be incandescent with rage, particularly if it looked like the author had quit writing the main series.
Keep my mouth shut until I read it.