When submitting a book proposal to an agent or publisher, you need to include sample chapters. Most often they ask for three sample chapters, but some want only one and some as many as you have written. For that reason it's important to check their writers' guidelines to determine the appropriate number for each submission. With fiction, they will usually want the first three chapters. With nonfiction, it's the first chapter, plus two more of your choice. It is recommended that one of those be the best one or the one that contains the essence of your book. The other one might be the most “risky” chapter, if there happens to be one. That means one that the agent or publisher might disagree with or question for any reason. If there are no controversial chapters, then the third chapter can be any other one you wish. For more on sending sample chapters, go to: http://www.writersdigest.com/editor-blogs/questions-and-quandaries/which-sample-chapters-should-you-send-to-agents.