When you think you're downloading a whole e-book in a series, but it's really just a 50 page or so "backstory". I was very sad when I turned a page in book 4 to what I thoght was a new chapter, only to find it was the first page of the first book.
Well...if the book is #2 or later in a series, the author has to give some sort of backstory. Who are these people, and how did they get in the predicament that they're in? Either the author can summarize it as a prologue, or they can drop it in expository chunks here and there.
As a reader, I prefer the prologue, because usually I've just finished the first book(s) or I go back and read the previous book(s) again. As a writer, I still prefer the prologue, if only to escape the dreaded "As You Know, Bob" exposition, where the characters tell each other things they already know. "As you know, Bob, this wizard appeared on the doorstep and told me that the ring my grandfather gave me is a Magical MacGuffin." "Yes, I remember, Jim, and then the unicorns showed up and..."
Some writers/editors say that you should remind the reader of salient points here and there, things that happened earlier in the same book
, because by the time the reader has gotten to chapter 5 they could have forgotten what happened in chapter 1. How you could forget the wizard, the Magical MacGuffin Ring, the unicorns, and Our Heroes Bob and Jim, I'm not sure, unless you're reading the novel one or two sentences at a time. (HELP,! I've, gone, into, comma, overload,!)