Name some examples of figurative language used in "A Separate Peace." How does it add to our understanding of the book ?

One of the best examples of figurative language enhancing our understanding of the book comes in at the very end of the novel, when Leper is describing the events at the tree.  Brinker brings him in to describe what happened on that infamous day, and in his description, Leper uses figurative language techniques.  This serves to enhance the descriptiveness and help the reader to imagine it in their heads, and it also completely clarifies the events that occurred that day. So, it not only makes the writing more descriptive and beautiful, it also helps the reader see the action, and understand the events that occurred.  Leper starts by saying that he could see the dark profiles of Gene and Finny on the branch, with the sun shining around them.  He describes the sun using a simile (comparing two things using the words like or as):

"the rays of the sun were shooting past them, millions of rays shooting past them like--like golden machine-gun fire."

Here, he uses a simile to compare the sun shining around them to the firing of golden bullets streaming from a machine-gun.  Having been just released from war training, this description not only lets us know how heavily the war weighs on Leper's mind, but it also paints a good image for us to picture.  He continues to say that their profiles looked "as black as death," using another simile.  This simile is very grave, referring the two boys to death; it's a sinister comparison, adding an air of seriousness to the proceedings, and of foreshadowing to the coming events.

Then, the fateful event of Finny falling.  Leper describes it with another simile, saying that the two boys moved "like an engine...first one piston sinks, and the the next one sinks."  This indicated that Gene jostled the branch down first, which had a chain-reaction, and made Finny sink too, and fall.  It clearly implicates Gene in purposefully knocking Finny off of the tree, and does so using a comparison to a two-piston engine, which helps us to picture it in our heads.

Leper's similes is one example of how Knowles uses figurative language to be descriptive, to help the reader picture things in their minds, and to clarify our understanding of the events.  I hope that helps a bit--good luck!