Nature by Ralph Waldo Emerson

Nature tells of one man's philosophy, which is centered around, well, nature.  Ralph Waldo Emerson believed that through observing nature as a child does, somebody could find happiness and meaning in their life.  The book centers around the untouched qualities of nature and the unimaginable qualities of God.  Understanding the first one is the only way to get a glimpse of the second.

This is the work that inspired transcendentalism.  Thoreau and others all got their inspiration from Nature.  Whether you agree or disagree with its philosophy, Nature is an important read, historically.