To quote or not to quote

You know those little quotes at the beginning of books, usually from a different writer or a song lyric? I've been thinking a lot about them lately. I used one in the last MS I turned in to my agent, and I'm using another one for this book.

One example that stuck out to me was in Jeri Smith-Ready's YA novel SHADE, which opened with an Edgar Allan Poe quote from "The Premature Burial."

The boundaries which divide Life from Death are at best shadowy and vague.

Who shall say where the one ends, and where the other begins?

It's a perfect taste of what is to come in Ready's novel, which is a wonderful ghost story. I like the introductory quote. In a way, these quotes are almost prologue-like. They set the tone and should be kept in mind for the duration of the story.

Now you might run into copyright issues with using quoted material, so make sure you have the right to use it or choose work that is already public domain.

What about you? Do you read the introductory quotes or skip them? What purpose do you think they serve?