When writing a book synopsis, introduce the most relevant characters and explain how their central conflicts drive the overarching plot. Describe the most significant plot points, but don't include subplots that aren't essential to understanding the story. Explain the climactic resolutions to any major conflicts you previously discussed.
Depending on your preference, you may want to start by writing a brief opening paragraph in third person summarizing the overarching plot. Alternatively, you can leave this section out and immediately describe the setting, the main character's personality and the characters' circumstances at the beginning of the story. If you're writing about an ensemble cast in different locations, conserve words by putting short tags before the character description, such as "In Manhattan: Avery Lemon is..."
Write about the plot chronologically, and include a clear beginning, middle and end. Briefly explain character relationships and the connections between their conflicts. Whenever possible, try to convey the same tone as the original book, so readers understand how to interpret the character interactions and environments. An effective synopsis explicitly states each major character's behavioral or emotional arc. For example, a character who is afraid to go outside for more than five minutes at a time might challenge those fears when a person he cares for is in danger. Short quotations from the book aren't necessary, but are often helpful for establishing tone and helping the reader connect with a character.