Thoreau as revealed in his journal; Thoreau in the Mountains, by Henry David Thoreau, commentary by William Howarth. New York: Farrar, Straus, and Giroux Inc. 373 pp. $15.

If all you've read by Henry David Thoreau is ''Walden'' plus an essay or two, you've missed a lot. Some of his best work can be found in his diary or ''Journal,'' which wasn't published in his lifetime and still goes largely unnoticed. At least that's the opinion of dditor Howarth, professor at Princeton University and specialist on Thoreau.

Portions of Thoreau's Journal (1839-1862), chronicling his backpacking expeditions through the mountains of New England, are reproduced at length here. The accounts reflect the writer's changing attitudes and interests over the years, and vary greatly in readability. There are moments of magic, but more often there are lists of the birds Thoreau spotted, of the plant species he saw, of the Indian words he learned, of the food he brought. The accounts are illustrated with period woodcuts and capably spliced together with commentary from Howarth, who has retraced Thoreau's steps in person.