I found an internet site that had links to nature and literature course syllabi. This is a large selection of books that are offered on various courses. I didn't select anything already listed in this thread. I also didn't read any of these yet, so I can't vouch for them. Professors of literature chose them for their courses, though, if that means anything.
McPhee, Coming into the Country
Zwinger, Run, River, Run
Terry Tempest Williams, Refuge
Nelson, The Island Within
William Bartram, Travels
Richard Llewellyn, How Green Was My Valley
Gerald Durrell, My Family and Other Animals
Washington Irving, The Sketch Book
Annie Dillard, Pilgrim at Tinker Creek, Teaching a Stone to Talk
Loren Eiseley, The Immense Journey
Scott Russell Sanders, Staying Put
Bill Devall and George Sessions, Deep Ecology
Mary Oliver, American Primitive
Diane Ackerman, The Moon by Whale Light
Anaya, Bless Me, Ultima
Anton Chekhov, The Cherry Orchard
N. Scott Momaday, The Way to Rainy Mountain
Marcia Bonta, Appalachian Spring
Joseph Meeker, The Comedy of Survival
Annette Kolodny, The Lay of the Land
Elizabeth Gray, Green Paradise Lost
Craig Lesley, Riversong, Winterkill
Louise Erdrich, Tracks
Marilynne Robinson, Housekeeping
A. B. Guthrie, Big Sky
James Welch, Fool's Crow
Wendell Berry, Remembering, The Memory of Old Jack
Anne Matthews, Where the Buffalo Roam
Al Stewart, The Holy Season, Walking in the Wild
Ernest Gaines, A Gathering of Old Men
James Dickey, Deliverance
Ed. David R. Pichaske, Late Harvest: Rural American Writing
Edward O. Wilson, Biophilia
Bill McKibben, The End of Nature
Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900
Yi-fu Tuan, Topophilia