He is perhaps best known now for A Sand County Almanac. The book was published posthumously. Eco Watch has a page on Leopold which includes some quotations from this book:
"The land ethic simply enlarges the boundaries of the community to include soils, waters, plants, and animals; or collectively: the land. ... In short, a land ethic changes the role of Homo sapiens from conqueror of the land-community to plain member and citizen of it."
"Conservation is a state of harmony between men and land"
"Recreational development is a job not of building roads into lovely country, but of building receptivity into the still unlovely human mind."
The New York Times describes it as "still an innovative book, a mix of field notes, meditations and a naturalist’s credo." Yes Magazine has a review, which notes that
The book was little noticed until 20 years later, during the environmental awakening of the 1970s, when a paperback edition turned into a surprise bestseller. Now, 50 years later, the book is high on the most-beloved list of environmentalists...