One important thing is to capture the essence of the story. Let's say for 'To Kill a Mockingbird'. You have to include Scout's growth as a character, the trial of Tom Robinson which is a major event highlighting racial injustice, and the overall theme of innocence lost. Another aspect is to keep it brief but still convey the emotional impact of the novel. You can't go into all the details but pick the most impactful ones like the relationship between Atticus and his children which is key to understanding the story.
First, you need to identify the main characters. For example, in 'The Great Gatsby', it's Jay Gatsby, Daisy, and Nick. Then, focus on the central plot. In this case, Gatsby's pursuit of Daisy and the illusions of the American Dream. Also, don't forget the main themes like wealth, love, and class. These elements will help create a concise yet comprehensive short digest.