The interesting thing about Thoreau is that, like Herman Mellville, he was almost totally ignored during his own time. His first book, A Week on the Concord and Merrimack Rivers, completely bombed. The publisher eventually returned about 700 copies of the 1000 copy printing run. That evening, Thoreau wrote in his journal:
I have now a library of nearly nine hundred volumes, over seven hundred of which I wrote myself.
After this failure, Thoreau spent five years crafting his masterpiece, Walden. But this, too, was a failure. When he died in 1862, Thoreau was considered less than even a footnote to the "great" naturalists: Emerson, Irving, Longfellow, Lowell, and Dr. Homes. Yet today, all except Emerson are largely forgotten.

Here are some of my favorite Thoreau quotes (from Walden):
Simplicity, simplicity, simplicity!

If you have built castles in the air, your work need not be lost; that is where they should be. Now put foundations under them. (256)

Go fish and hunt far and wide day by day -- farther and wider -- and rest thee by many brooks and hearth-sides without misgiving. Remember thy Creator in the days of thy youth. Rise free from care before the dawn, and seek adventures. Let the [noon find thee by other lakes, and the night overtake thee everywhere at home. There are no larger fields than these, no worthier games than may here be played. Grow wild according to thy nature, like these sedges and brakes, which will never become English bay. Let the thunder rumble; what if it threaten ruin to farmers' crops? That is not its errand to thee. Take shelter under the cloud, while they flee to carts and sheds. Let not to get a living be thy trade, but thy sport. Enjoy the land, but own it not. Through want of enterprise and faith men are where they are, buying and selling, and spending their lives like serfs. (166)

I should not talk so much about myself if there were anybody else whom I knew as well. (1)

If my jacket and trousers, my hat and shoes, are fit to worship God in, they will do (17)

It certainly is better to accept the advantages, though so dearly bought, which the invention and industry of mankind offer. (31)

No doubt another may also think for me; but it is not therefore desirable that he should do so to the exclusion of my thinking for myself. (36)

Tuition, for instance, is an important item in the term bill, while for the far more valuable education which he gets by associating with the most cultivated of his contemporaries no charge is made. (39)

Our inventions are wont to be pretty toys, which distract our attention from serious things. They are but improved means to an unimproved end. (41)

We are in great haste to construct a magnetic telegraph from Maine to Texas; but Maine and Texas, it may be, have nothing important to communicate. Either is in such a predicament as the man who was earnest to be introduced to a distinguished deaf woman, but when he was presented, and one end of her ear trumpet was put into his hand, had nothing to say. As if the main object were to talk fast and not to talk sensibly. We are eager to tunnel under the Atlantic and bring the Old World some weeks nearer to the New; but perchance the first news that will leak through into the broad, flapping American ear will be that the Princess Adelaide has the whooping cough. After all, the man whose horse trots a mile in a minute does not carry the most important messages; he is not an evangelist, nor does he come round eating locusts and wild honey. I doubt if Flying Childers ever carried a peck of corn to mill. (41)

I should never have broken a horse or bull and taken him to board for any work he might do for me, for fear I should become a horseman or a herds-man merely (45)

As I did not teach for the good of my fellow-men, but simply for livelihood, this was a failure. (55)

For myself I found that the occupation of a day-laborer was the most independent of any, especially as it required only thirty to forty days in a year to support one. (56)

We may not arrive at our port within a calculable period, but we would preserve the true course. (57)

Probably I should not consciously and deliberately forsake my particular calling to do the good which society demands of me, to save the universe from annihilation; and I believe that a like but infinitely greater steadfastness elsewhere is all that now preserves it. (58)

I read in the Gulistan, or Flower Garden, of Sheik Sadi of Shiraz, that "they asked a wise man, saying: Of the many celebrated trees which the Most High God has created lofty and umbrageous, they call none azad, or free, excepting the cypress, which bears no fruit; what mystery is there in this? He replied: Each has its appropriate produce, and appointed season, during the continuance of which it is fresh and blooming, and during their absence dry and withered; to neither of which states is the cypress exposed, being always flourishing; and of this nature are the azads, or religious independents.—Fix not thy heart on that which is transitory; for the Dijlah, or Tigris, will continue to flow through Bagdad after the race of caliphs is extinct: if thy hand has plenty, be liberal as the date tree; but if it affords nothing to give away, be an azad, or free man, like the cypress." (63)

The intellect is a cleaver; it discerns and rifts its way into the secret of things. (79)

If it should continue so long as to cause the seeds to rot in the ground and destroy the potatoes in the low lands, it would still be good for the grass on the uplands, and, being good for the grass, it would be good for me. (105)

I have found that no exertion of the legs can bring two minds much nearer to one another (106)

The value of a man is not in his skin, that we should touch him. (109)

I have a great deal of company in my house; especially in the morning, when nobody calls. (109)

Shall I not have intelligence with the earth? Am I not partly leaves and vegetable mould myself? (110)

The highest that he appeared to conceive of was a simple expediency, such as you might expect an animal to appreciate; and this, practically, is true of most men. (119)

The true husbandman will cease from anxiety, as the squirrels manifest no concern whether the woods will bear the chestnuts this year or not, and finish his labor with every day, relinquishing all claim to the produce of his fields, and sacrificing in his mind not only his first but his last fruits also. (133)

Not till we are lost, in other words not till we have lost the world, do we begin to find ourselves, and realize where we are and the infinite extent of our relations. (137)

I am convinced, that if all men were to live as simply as I then did, thieving and robbery would be unknown. These take place only in communities where some have got more than is sufficient while others have not enough. (138)

Give me the poverty that enjoys true wealth. (157) But the only true America is that country where you are at liberty to pursue such a mode of life as may enable you to do without luxuries, and where the state does not endeavor to compel you to sustain the slavery and war and other superfluous expenses which directly or indirectly result from the use of such things. (164)

I should be glad if all the meadows of the earth were left in a wild state, if that were the consequence of men’s beginning to redeem themselves. (164)

I found in myself, and still find, an instinct toward a higher, or, as it is named, spiritual life, as do most men, and another toward a primitive rank and savage one, and I reverence them both. (167)

No humane being, past the thoughtless age of boyhood, will wantonly murder any creature which holds its life by the same tenure that he does. (169)

Whatever my own practice may be, I have no doubt that it is a part of the destiny of the human race, in its gradual improvement, to leave off eating animals, as surely as the savage tribes have left off eating each other when they came in contact with the more civilized. (172)

If one listens to the faintest but constant suggestions of his genius, which are certainly true, he sees not to what extremes, or even insanity, it may lead him; and yet that way, as he grows more resolute and faithful, his road lies. The faintest assured objection which one healthy man feels will at length prevail over the arguments and customs of mankind. (172)

The laws of the universe are not indifferent, but are forever on the side of the most sensitive. (174)

We are conscious of an animal in us, which awakens in proportion as our higher nature slumbers. It is reptile and sensual, and perhaps cannot be wholly expelled; like the worms which, even in life and health, occupy our bodies. Possibly we may withdraw from it, but never change its nature. I fear that it may enjoy a certain health of its own; that we may be well, yet not pure. The other day I picked up the lower jaw of a hog, with white and sound teeth and tusks, which suggested that there was an animal health and vigor distinct from the spiritual. This creature succeeded by other means than temperance and purity. "That in which men differ from brute beasts," says Mencius, "is a thing very inconsiderable; the common herd lose it very soon; superior men preserve it carefully." Who knows what sort of life would result if we had attained to purity? If I knew so wise a man as could teach me purity I would go to seek him forthwith. "A command over our passions, and over the external senses of the body, and good acts, are declared by the Ved to be indispensable in the mind's approximation to God." Yet the spirit can for the time pervade and control every member and function of the body, and transmute what in form is the grossest sensuality into purity and devotion. The generative energy, which, when we are loose, dissipates and makes us unclean, when we are continent invigorates and inspires us. Chastity is the flowering of man; and what are called Genius, Heroism, Holiness, and the like, are but various fruits which succeed it. Man flows at once to God when the channel of purity is open. By turns our purity inspires and our impurity casts us down. He is blessed who is assured that the animal is dying out in him day by day, and the divine being established. Perhaps there is none but has cause for shame on account of the inferior and brutish nature to which he is allied. I fear that we are such gods or demigods only as fauns and satyrs, the divine allied to beasts, the creatures of appetite, and that, to some extent, our very life is our disgrace. (175)

From exertion come wisdom and purity; from sloth ignorance and sensuality. (176)

Heaven is under our feet as well as over our heads. (225)

The imagination, give it the least license, dives deeper and soars higher than Nature goes. (228)

If we knew all the laws of Nature, we should need only one fact, or the description of one actual phenomenon, to infer all the particular results at that point. Now we know only a few laws, and our result is vitiated, not, of course, by any confusion or irregularity in Nature, but by our ignorance of essential elements in the calculation. Our notions of law and harmony are commonly confined to those instances which we detect; but the harmony which results from a far greater number of seemingly conflicting, but really concurring, laws, which we have not detected, is still more wonderful. The particular laws are as our points of view, as, to the traveler, a mountain outline varies with every step, and it has an infinite number of profiles, though absolutely but one form. Even when cleft or bored through it is not comprehended in its entireness. (230)

Why is it that a bucket of water soon becomes putrid, but frozen remains sweet forever? It is commonly said that this is the difference between the affections and the intellect. (235)

We should be blessed if we lived in the present always, and took advantage of every accident that befell us, like the grass which confesses the influence of the slightest due that falls on it; and did not spend our time in atoning for the neglect of past opportunities, which we call doing our duty. We loiter in winter when it is already spring. (249)

The universe is wider than our views of it. (253)

If one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours. (256)

Let every one mind his own business, and endeavor to be what he was made. (258)

However mean your life is, meet it and live it; do not shunt it and call it hard names. (259)

Drive a nail home and clinch it so faithfully that you can wake up in the night and think of your work with satisfaction.—a work at which you would not be ashamed to invoke the Muse. (261)