Henry David Thoreau is perhaps best known as the author of ''Walden,'' a book about the woods and water around Concord, Mass., where he spent most of his 44 years living a simple life.

Henry David Thoreau

He often is regarded as the classic New England crank, a rigid and mostly humorless fellow who never had a relationship with any woman that was not sisterly and whose principled manner sometimes could be mistaken for arrogance.

But that is only half right.

Thoreau, who was born in 1817 and graduated from Harvard in 1837, was a skilled land surveyor and botanist, an inspired writer and a profound thinker whose ideas about art, science, nature and even politics are still relevant today.

His essay ''Civil Disobedience,'' about being jailed briefly for refusing to pay a tax that supported the Mexican War, influenced the likes of Mahatma Gandhi, Martin Luther King Jr. and John F. Kennedy.

Thoreau's writings on nature, however, remain his most important works. His keen observations of the outdoors - not only in ''Walden'' but also in ''A Week on the Concord and Merrimack Rivers,'' ''Cape Cod'' and ''The Maine Woods'' - have contributed greatly to our understanding of the natural world and man's place in it.

''The word 'ecology' didn't even appear in print until 1870 in Germany - after Thoreau's death,'' said Richard T. Forman, a professor of landscape ecology at Harvard. ''Thoreau was a generation and a half ahead of his time. He looked deeply into nature, and he tried to understand the changes he saw. And he saw things no one else did.''

Thoreau's views on nature were shaped by his transcendentalism - a belief he shared with friend and mentor Ralph Waldo Emerson that God is present in both man and nature, and may be revealed through intuition and self-reflection. He subscribed to the transcendentalist notion of a single universal soul that underlies all of nature.

''It is as immortal as I am,'' Thoreau writes about a tall pine in ''The Maine Woods,'' ''and perchance will go to as high a heaven, there to tower above me still.''

But Thoreau was not merely a dreamer or a bookworm. He sought to experience nature, not just comment on it. In all kinds of weather, he walked - or ''sauntered'' - in the Concord woods, inspecting up-close the flora and fauna of the place.

''He knew the country like a hawk or a fox,'' Emerson wrote after Thoreau's death. ''Under his arm he carried an old music book to press plants; in his pocket, his diary and pencil, a spy glass for birds, microscope, jack-knife and twine.''

In 1845, Thoreau moved into the now-famous 10-by-15-foot house he built - for $28.12 - on the shore of Walden Pond. It was his retreat, he said, a place to ''transact some private business with the fewest obstacles'' - namely, to write his first book, ''A Week on the Concord and Merrimack Rivers.''

A year into his stay at Walden, Thoreau left for the first of his three visits to the Maine woods. He had been here in 1838 looking for a teaching position, but this was different. He was looking now for the frontier, a wilder nature than he could find in Concord - he had traveled northeast to go west.

Thoreau began and ended each of his Maine trips at the Bangor home of his cousin-in-law, George Thatcher, a businessman who served as his travel agent and, on the first two visits, his escort.

In 1846, Thoreau traveled to and from Mount Katahdin on the Penobscot River. In 1853, he went to Greenville, across Moosehead Lake and down the West Branch of the Penobscot River to Chesuncook Lake. His final trip, in 1857, was his longest, taking him to Chesuncook, Chamberlain and Eagle lakes on the Allagash River, across Webster Stream, down the East Branch of the Penobscot and back to Bangor.

The essays based on those trips - ''Ktaadn,'' ''Chesuncook'' and ''The Allegash and East Branch'' - were published separately and later combined in ''The Maine Woods.''

The effect of those visits on Thoreau was significant: The author's last words before succumbing to tuberculosis in 1862 were ''moose'' and ''Indians.'' Even casual readers of ''The Maine Woods'' will discern the special fondness he had for both.

Thoreau had come to Maine to see not only its untamed forest and rivers but also its people - the pioneers, the loggers and landowners and, most of all, the American Indians.

During his lifetime, Thoreau filled 11 notebooks with material from the many books and correspondence he had read about American Indians. The so-called ''Indian Notebooks'' also include some of Thoreau's impressions of the American Indians he encountered in Massachusetts, Maine and Minnesota, where, at the end of his life, he met the war chief Little Crow.

Of course, Thoreau was not disappointed by what he found here. He was barely out of Bangor when he was struck by the beauty of the state. ''I began to be exhilarated by the sight of the wild fir and spruce tops, and those of other primitive evergreens, peering through the mist in the horizon,'' he writes. ''It was like the sight and odor of cake to a schoolboy.''

Thoreau was overwhelmed by Katahdin, stirred by Chesuncook, tested by the Mud Pond carry. And he was impressed with the ''intelligence and refinement'' of the people he encountered, whom he compared favorably to the ''helpless multitudes'' of Concord who ''depend on gratifying the extremely artificial wants of society.''

''The Maine Woods,'' the work of one of America's finest writers and one of its first naturalists, shows the state as it was then - a vast wilderness on the brink of discovery and development.