Irony – Modern American literature is unimaginable without this idea, and it comes in many forms and flavors in the works we read.
The starting point for most forms of literary irony is a gap between what the words in a text seem to be saying, and the possible (often very contradictory) meanings that lie behind those words, and that a good reader will be aware of. When Huck says, for example, that he'll "go to hell" for the "awful" sin of helping Jim escape slavery, and believes it himself, Mark Twain's reader understands that he is "really" doing the right thing.
Adventures of Huckleberry Finn is throughout an ironic first-person narrative (or an unreliable first-person narrative): Huck never himself understands what his story is revealing to us, about himself, for example (he's not a "low-down, ornery" no-good kid), or about Emmeline's poetry (to him it's "very good poetry"), or about slavery. Modern writers are even more likely to use third-person ironic narration as the technique for telling their stories (see HENRY JAMES). Among lots of other things, irony means readers have to work harder, and writers have a way to make the meaninglessness of modernity meaningful.
James, Henry – O no you don't. You've got to see Henry James' work for yourself.