Writing a book summary may seem simple -- if you take that to mean simply regurgitating the events within a story. However, it's important to not only discuss the events of a story but also demonstrate understanding of how the events are interrelated and driven by the characters involved. When summarizing the events in a story, focus on the main points of the narrative arc.
Summarize the Exposition
The exposition is simply the beginning of a story, in which the author "sets the stage" for the events to come. The characters and setting are introduced, and the main conflict of the story is hinted at. For example, a summary of the exposition of "The Great Gatsby" could read, "A young businessman, Nick Carraway, moves to Long Island in the 1920s and meets Jay Gatsby, a rich bachelor with a mysterious past." Introducing the main characters, the setting and the plot allows the reader to understand the main context of the story.
Define the Inciting Incident
After a summary discusses the exposition, shift the focus to the inciting incident and the rising action within the story. The inciting incident is a singular event that "kicks off" the story and leads to the major conflict within the novel. This leads to the rising action, in which the story continues to build and eventually comes to a point where the main character might have to take drastic action -- or might miss her opportunity to do this.
For example, in Shakespeare's "Hamlet," the Danish prince discovers that his father was murdered by his own brother, which leads to the infamously tragic events to come. The revelations of the inciting incident and rising action result in events that may alter the future in unchangeable ways. A comprehensive plot summary defines the inciting incident, briefly describes it and outlines the events that lead to the highest point of action.
Discuss the Climax
All stories eventually reach a "point of no return," the climax. The climax is an event that changes the course of a story, for better or worse. For example, the climax of Shakespeare's "Romeo and Juliet" happens when Romeo murders Juliet's cousin, Tybalt, in a duel. In your plot summary, define the consequences or results of this point in the plot. Although Romeo and Juliet's romance had been forbidden, it becomes near impossible to maintain after Tybalt's death, since Romeo is banished to Mantua. His banishment furthers the symbolic divide between the two feuding families. The climax often changes the characters and can set off a chain reaction of events.
Tie It Up
The falling action of a story is the "fall out" that comes as a result of the climax -- the chain reaction. In "Romeo and Juliet," the falling action is so dramatic -- the tragic suicides of the young lovers -- that people might think this event is the climax; however, this famous tragic scene is the result of the events triggered in the duel of the climax. The plot's resolution is not always as "happily ever after" as the phrase suggests. The resolution show how characters respond to the events that transpired earlier in the narrative arc.
For example, as the curtains close at the end of "Romeo and Juliet," the two families vow to end their feud. Although this does not bring their children back to life, it suggests a social change brought about by tragic loss. In your plot summary, explain how characters respond to the events of the story -- and what, if any, lessons they may have taken away from the experiences.