Every book has a theme and often they contain more than one. In order to successfully identify and write about a book's central theme, you need to closely read the text, plan before you write and provide sufficient analysis and direct quotes from the story to support your point.
Literary themes are implied through elements of the text, so taking notes as you read helps you identify and gather the book's clues. Underlining interesting passages, jotting down questions, marking moments when characters have to make important or difficult decisions and recording instances of symbolism and foreshadowing will deepen your interpretation of the book while also building a toolbox of material to draw from when you write your essay.
Find the Theme
A theme is a universal idea that is often a reflection on human experience suggested by the text. Often a theme can be gleaned from an evaluation of the circumstances surrounding repeated objects or symbols in the story. The central theme can also be identified by examining the characters' strengths, weaknesses, values, thoughts and actions. The theme is not explicitly stated, so your notes are crucial to identifying and evaluating the book's central theme. Examples of book themes include "death is a part of life," "crime doesn't pay," "all humans need to be loved," "aging is inevitable" and "racism harms communities for generations."
When you have determined the theme of the book, you can formulate a thesis statement. The thesis statement conveys a particular point about the theme and how it is manifested in the book. For example, you might identify the theme of Paulo Coelho's book "The Alchemist" as "the treasure of human potential." Using that theme, you can write the thesis statement, "Santiago's journey for external treasure was necessary to discovering the treasure that lay inside him."
An essay on the theme of a book is a synthesis of your reflections on elements of the story and their relationship to a deeper and broader meaning. The analysis shows how the theme is illustrated in the text and why the theme matters, both in story and in the real world. Your analysis can examine why characters make specific choices, how the book's setting impacts the plot, what conflicts exist in the text and how those conflicts are resolved. Most importantly, your analysis should answer the question of why any of these points matter.
In addition to analysis, the body of the essay should incorporate evidence to support the the analysis. Using direct quotes from the novel, you can concretely link the theme to the story with specific examples. The quotes prove the point you are making at that moment. All quotes must be properly cited and must be accompanied with explanations of how specifically the evidence is a concrete manifestation of the book's overall theme, as well as the essay's overall point.