If ever there was a uniquely gifted observer of the natural world, it's Henry David Thoreau, and he knew it early.
Here he is at 27:
"Sometimes, when I compare myself with other men, methinks I am favored by the gods. They seem to whisper joy to me beyond my deserts, and that I do have a solid warrant and surety at their hands, which my fellows do not."
This sense of specialness is what makes him not only so fascinating, and so informative and interesting, but also so frustrating. He felt a real genuine oneness with nature; I think he much preferred nature over people, and to read Walden again is to spend time with both a marvelous teacher and an arrogant shit, a prig, a puritan; self-aware but also self-involved and self-important. Solipsist and misanthropic environmentalist.
Like Whitman and, perhaps, everybody, Thoreau was his own best subject. He knew himself, and not only that he was interested in discovering it, recording, attentive to his moods, although I don't think he ever stood outside himself that much, ever saw himself in perspective; maybe it would have shut him up.
"I should not talk about myself if there were anybody else whom I knew as well. Unfortunately, I am confined to this theme by the narrowness of my experience."
If you just glance at his life, no one would doubt that his actual experience probably was narrow; he didn't live the big life of a writer, like, say, Melville. But he made that narrowness expand.