I'd recommend checking out Donald Hughes, What is Environmental History? It has an excellent run down of the evolution of the field and some of the formative works and voices as well as some of the branches that have evolved. Hughes does cover some of the reason you may find the field hard to pin down-- as the umbrella term 'environmental history' seems to cover a vast number of approaches and theoretical concerns, though in general they all touch on interaction/intersections between humans & environment/ecology/landscape.

Otherwise, some key works from key voices you may want to check out.

• Alfred Crosby, Ecological Imperialism: The Biological Expansion of Europe, 900-1900.

Crosby's work is basically what if Jared Diamond's GGS focused on more individual case studies and spent more time considering the relationship between colonialism by humans & accompanying species (or 'portmanteau biota' as Crosby puts it). Crosby's book predates GGS by about 20 years I believe and one of my chief qualms with GGS was always that Crosby's name is mentioned no where when its readily apparent that he is leaning on Crosby and the literature that followed his book.

• William Cronon; Changes in the Land: Indians, Colonists, and the Ecology of New England.

Cronon's book was definitely foundational-- and to think it started as a course term paper. It is easily accessible and like Crosby tackled the intersection of colonial/environmental changes. It also dealt a great deal more with how individuals (or societies) see the environment, specifically how European settlers could not see the ways in which indigenous communities has transformed the Northeastern Woodlands because their land use patterns were unfamiliar to those European peasants. While the book is great and very readable, I would actually recommend "Nature’s Metropolis: Chicago and the Great West" as Cronon's best contribution to environmental history. It is way fatter (longer) and more technical, but I think he has a lot of great insights that I cite in my own work.

• J.R. McNeill; Something New Under the Sun: An Environmental History of the Twentieth Century World.

A great book from the second generation of Env. History scholars, though J.R. McNeill is at this point an elder statesman in the field. The book covers the development of the modern city and tracks large scale shifts in the way resources have been used or thought about (specifically tracking the transition to oil). Also really good would be his earlier work (which I like a bit more tbh); "Mosquito Empires: Ecology and War in the Greater Caribbean, 1620-1914"

• Donald Worster, Dust Bowl: The Southern Plains in the 1930’s.

Worster is another foundational figure. Really any of his books could fit in as a good introduction to him as he was particularly prolific.

I would also add in a few other works from the second/third generation of scholars that made fair impacts on the field, though this list is hardly complete and inclusive of everything important.

• Arun Agrawal, Environmentality; Technologies of Government and the Making of Subjects.

• Thomas G. Andrews, Killing for Coal: America’s Deadliest Labor War

• David Blackbourn, The Conquest of Nature: Water, Landscape, and the Making of Modern Germany

• Richard Grove, Green Imperialism: Colonial Expansion, Tropical Island Edens and the Origins of Environmentalism, 1600-1860

• James C. McCann, Maize and Grace: Africa’s Encounter with a New World Crop 1500-2000

• Chris Coggins, The Tiger and the Pangolin: Nature, Culture, and Conservation in China

• Peter Boomgaard, Frontiers of Fear: Tigers and People in the Malay World, 1600-1950

• Caroline Merchant, American Environmental History or Ecological Revolutions: Nature, Gender, and Science in New England

• William McNeill, Plagues and Peoples

Though the very last one should probably be up at the top as William McNeill (father of JR) was definitely a founding figure in Environmental history along with Worster, Crosby, and maybe Cronon.