Boone, Joseph Allen. "Delving and Diving for Truth: Breaking through to Bottom in Thoreau's Walden" ESQ 27, No. 3 (3rd Quarter 1981): 135-46.
Studies the numerous references to surfaces and depths in Walden.
Boudreau, Gordon V. The Roots of "Walden" and the Tree of Life. Nashville: Vanderbilt University Press, 1990, 241 p.
Traces the roots of Walden and its author's "radical genius."
Brooks, Van Wyck. "Thoreau at Walden." In The Flowering of New England, 1815-1865, pp. 359-73. New York: E. P. Dutton, 1937.
(The entire section is 988 words.)