Introduce the title and subject of your book in the first paragraph, along with a brief historic overview to establish the book's context. If your book is based on a recent news topic, you can introduce the book by giving a brief overview of the news event itself.
Write a chapter-by-chapter summary, dedicating several paragraphs to each chapter. Write in present tense, detail any research or interviews you did, and highlight the key points of each chapter and how it relates to other chapters. It may help you to use parenthetical notations.
Use details. The purpose of the synopsis is to sell your manuscript. Nonfiction manuscripts must demonstrate the same solid writing that good fiction manuscripts contain. Although you're writing a synopsis for nonfiction, write as if you're telling a story. Avoid dry writing and simply rattling off a string of facts. Even though nonfiction is strictly factual, publishers want to see that you can tell a cohesive story that will draw readers into it.
Demonstrate an intimate knowledge of and enthusiasm for your subject. Nonfiction is typically written best by writers who know and care deeply about their topic. Phrase your letter to demonstrate your confidence that you know your subject better than anyone else.