New Englander, Harvard graduate, pencil-maker, surveyor, naturalist, and writer — Emerson's younger Concord neighbor, less temperamentally optimistic than his mentor but in some ways a more consequential writer. Thoreau lived most of his 44 years in and around Concord, Massachusetts, kept a journal of extraordinary density for 24 years, and produced two small books that have done outsized work in American and world thought.
Walden (1854), the record of two years, two months, and two days he spent in a one-room cabin by Walden Pond on Emerson's land, is not primarily a nature book or a how-to of simple living, though it is both. It is a sustained experiment in seeing what is left of a life when one strips away the elaborations that "civilization" presents as necessary — work, property, newspapers, the hum of opinion — and attends instead to the ordinary particulars (loon, beanfield, ice, the sound of a train) that the elaborations usually screen out. "I went to the woods because I wished to live deliberately" is the most quoted sentence; "the mass of men lead lives of quiet desperation" is the darker one immediately before it.
Resistance to Civil Government (1849), later retitled Civil Disobedience, was written after Thoreau's night in jail for refusing to pay a poll tax supporting the Mexican War and slavery. Its argument — that the individual's first obligation is to conscience, not to law, and that unjust law must be actively disobeyed — became one of the few American political essays to have permanently shifted global political practice. Gandhi read it in South Africa; King read Gandhi; the line runs straight into every 20th-century mass non-violent movement.
His Journal (14 volumes in the Princeton edition) is arguably his real masterwork — proto-phenomenological observation carried on day after day without the shape of a book forcing its material.