Make a list of agents. Go to the bookstore and leaf through recent narrative non-fiction that is similar to yours. Authors often name their agents in the acknowledgements. You can cross-reference the names in an agent directory. See Resource section referenced below. Check agent names listed under the specific genre.
Network. Attend literary events, writing conferences and gatherings where agents will attend. When you register for a conference, you often have the option of paying an additional fee to meet with an agent for 20-30 minutes. If you have a finished, polished manuscript, sign up for a one-on-one.
Write a letter to an agent who handles works in narrative non-fiction and seeking new authors. Your one-page query should be three paragraphs: address why the subject is worth an entire book, a paragraph about your book, and a brief autobiography. Include your experience as a writer, achievements and promoting the book.
Be prepared to send sample chapters upon request from a prospective agent. Choose the chapters that you feel best represent the book. Make sure your writing is in top form.
Keep a log of communication with prospective agents. If an agent says he'll get back to you in three weeks, don't hesitate to follow up with a brief, pleasant email or letter. If the agent doesn't offer a time-frame, ask, "When should I expect to hear from you?"
Be prepared to promote your book. Publishers today do not promote and market new authors. An agent is more likely to take on a book if you have a "platform" and a "marketing plan" that uses your expertise, professional connections, position in the community and anything else that would shine a spotlight your book.