Coming off a discussion of what a "prologue" is, I'm concerned, again, about the confusion that seems to be hovering around the concept of it.
Many beginning authors seem to think that it's like the TV Guide description — only longer.
What they're actually providing is a "synopsis" — a summary of the book (up to a point) that typically appears on on the dust jacket, and is one kind of sales tool to get a buyer to take the book to the register and become that book's owner.
A "synopsis" is a mistake when it explains Every Single Thing a Book Is Trying to Do. And I've even seen some that give away the story's ending (disaster).
A "prologue" is "bait in the water" and a far more useful tool for selling an e-book: since it naturally appears in the book sample. The best "prologues" generate a "read" decision in the reader: as in "wow ... I need to read this".
Startling is better. Short is better. And, in many cases, no prologue at all is the best option.