Let's see. With Walden, Thoreau tells a story about his life in the first person. We're going to go ahead and call that an autobiography. Maybe it's not as thrilling as Rob Lowe's or Portia De Rossi's, but it's an autobiography nonetheless.
Thoreau being Thoreau, Walden is no ordinary autobiography. Yes, he talks about his day-to-day life, but he also includes a load of abstract and esoteric thoughts. He reflects on philosophical issues, he cites ancient Greek, Roman, and Chinese philosophies, he discusses poetry, expresses social satire, and, of course, he can never stop observing the natural world around him. A person's own words can tell us wonders about them and the world around them. What does Walden tell us about Thoreau? And if you wrote your autobiography today, what would you tell the world about yourself?