What are examples of flashback used in literature?
Wuthering Heights, by Emily Bronte.
Wuthering Heights is mostly flashback, so that's the most prominent example I can think of. Most of the exposition about Huck's dad in Huckleberry Finn is through flashback. There are quite a few flashbacks in The Catcher in the Rye; most of it is just Holden wandering around New York and thinking about things so he flashes back a lot to give exposition about various events that have occurred and people has known.
I don't know; those are the examples I can think of off the top of my head right now. I hope this helped, but I wasn't really sure what you were looking for, so in the future maybe try providing a little more information.