The text is from The Journal of Henry David Thoreau, ed. Bradford Torrey and Francis Allen, 14 vols. (Boston: Houghton Mifflin, 1906).
Each blog post is an excerpt from that day's entry in the Journal, and although not necessarily the complete entry, it is an integral and intact section thereof.
All contents may be copied for private and non-profit use with proper attribution to The Blog of Henry David Thoreau.
Update: Also recommended would be this extensive Thoreau site.