So, classics I would recommend include both fiction and non-fiction
Fiction
Monkey Wrench Gang

Non-fiction
The world without us

Have you got recommendations for the group?

A Sand County Almanac by Aldo Leopold
-a classic and still pertinent today, especially the land ethic.

The Dharma Bums by Jack Kerouac
-especially for the descriptions of climbing Desolation Peak and working as a fire lookout in the middle of nowhere

For a few by British authors that some of you Americans may not have heard of:

The Wild Places by Robert MacFarlane - A search for the UK's remaining wild places
Mountains of the Mind also by Robert Macfarlane - a study of human fascination with mountains and montane environments
Wildwood by Roger Deakin - Nature writing loosely based upon a love of trees
Crow Country by Mark Cocker - a study of corvids and particularly the rook

The Anthropology of Turquoise: Reflections on Desert, Sea, Stone, and Sky by Ellen Meloy

An Unspoken Hunger: Stories from the Field by Terry Tempest Williams

The Future of Ice: A Journey Into Cold by Gretel Ehrlich

Pilgrim at Tinker Creek by Annie Dillard

Teaching a Stone to Talk: Expeditions and Encounters by Annie Dillard

A Country Year Sue Hubbell
Beyond the Aspen Grove Ann Zwinger
Great Possessions David Kline
Wildlife in the Garden Gene Logsdon
Edwin Way Teale's series Autumn across America
North with the Spring Wandering through winter Journey into Summer
Watchers at the Pond Franklin Russell
The Immortal wilderness John Hay
Idle Weeds David Raines Wallace
Backtracking Ted Levin
There is a series of books that comes out yearly The best Science & Nature Writing which is a collection of short essays by various authors.