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A quote (called an epigraph) is added to the start of a book or a chapter when it adds an insight to the story. What kind of insight is up to you: it might be an additional understanding of events on a meta level, it can be foreshadowing, it can be extra information, etc. It is never a random quote found in google, since that adds no insight. The epigraph is as integral a part of the story you're writing as anything else. If it adds nothing, it shouldn't be there at all.

For example, Ernest Hemingway's For Whom the Bell Tolls starts with a quotation from John Donne:

No man is an Iland, intire of it selfe; every man is a peece of the Continent, a part of the maine; if a clod bee washed away by the Sea, Europe is the lesse, as well as if a Promontorie were, as well as if a Mannor of they friends or of thine owne were; any mans death diminishes me, because I am involved in Mankinde; And therefore never send to know for whom the bell tolls; It tolls for thee.

This epigraph should inform the reader's understanding of Hemingway's novel, convey that it is bigger than the story of one guy named Robert Jordan.

The theory of intertextuality assumes that all works of art are created within the artistic tradition or cultural discourse and refer to preceding works or art.

One aspect of this is sometimes called "inspiration", where one artist feels inspired to create a piece of art by another artwork. But intertextuality goes beyond inspiration in that it encompasses "schools" of artists addressing similar topics or artists working in the same age or cultural context and many other phenomena of similarities and relations between "texts" (where "text" is anything using a set of rules to express information, so a film or painting is also a "text" in this sense).

The epigraphs that you ask about are a way for an author to make explicit the connections to other texts that he wants to encourage the reader to consider in their reading.

But epigraphs have also become a fad so you might want to avoid them, especially if you aren't completely sure about how to use them.

When this is done well the quotes are almost always made up by the author and are quoting someone in the world of the book or are exerts from a diary or are news bulletins etc.

The purpose of them is to give snippets of insight into the world, show things that are happening or bits of history or character background that help set the scene. It is a way to add flavour and depth for those that care while letting those who don't skip over it. For example the book I just finished reading had a section of the lead character's father's diary or from their holy book or similar at the start of each chapter to help flesh out the world.

If it fits into your personal style then go for it, but don't try to force it if it doesn't.

This is a matter of opinion, it is done, but not by most.

In my opinion, I recommend against it. First for all the reasons @SteveJessop has outlined; but just as important, I don't do it because to me it seems pretentious, as if comparing your writing to theirs, and it seems like trying to borrow the fame of other great writers to make your own look better.

The only exception would be something like a quote from a source you invented; a fictional philosopher, politician, religious icon, or military generals or whatever in your own invented world. Then you own the copyright on it.

I always stick to my own writing, I don't try to borrow anybody else's to make mine seem better, my characters never quote anybody I did not also invent in their world.

Is this advisable for fiction novels?

As with most such questions, that depends entirely on the skills of the author. For what it's worth, I've seen it quite often. For instance, Jack Vance's novels (science-fiction /fantasy) often used entertaining quotes (from, admittedly, fictitious sources) to start chapters. It depends entirely on the effect you want to produce, and whether you have the chops to do it right. It was fairly common in 19th century novels, as well.

I would disagree with most commenters here (save WhatRoughBeast and user57423), and say it depends entirely on your writing style. Common sense dictates that anything and everything you add should make sense and serve a purpose, so then the only argument left is, "to what degree should it make sense?"

So it becomes a question of style; do you want to cause your readers to ponder, to feel wonder or suspense or about the following paragraphs they are about to read, or are you just trying to add somebody else's cool prose to make yours look better? I would argue that all reasons are valid reasons, after all your novel should be a piece of art, or else why bother?

I really like the way Katherine Neville used quotes to kick off each wonderful chapter of great masterpiece, "The Eight." If your writing style can entertain one half as much as hers does, then you can't go wrong,

If you are clever enough, do it. I wrote a trilogy where I put a quote at the start of every chapter. The quotes were relevant to the theme or action of the chapter, but also carried a second purpose: they were quotes from works of great literature being read by a literature professor to help her through her grief. She was reading them in the hospital to her comatose husband. Her reading of those words entered into his consciousness and influenced his adventures in the realm of the dead as he fought his way back to the world of the living. I just didn't tell the reader of the connection until near the end of the story. She was his guide to safety, like Beatrice in Dante's Inferno. Since Dante was a major influence on my story, it fit. Don't just do it for stylistic reasons; make sure it fits the tale.