The internet is a fantastic resource for people in any field, and environmentalism is no different. To just give one example, there’s a library’s worth of environmental texts available as free E-books on Project Gutenberg that largely goes under the radar. Project Gutenberg is an archive of free E-books sponsored by UNC-Chapel Hill that allows people to download most works written before copyright or works whose copyrights have expired and therefore are in the public domain.
For this reason there are a lot of texts from early environmentalists and people who founded the movements in America:
John Muir’s writings are there including his most famous work My First Summer in the Sierra.
There are two works by Gifford Pinchot who, regardless of how you feel about his legacy, is an incredibly important historic figure, given that he was the first chief of the US Forest Service.
And if you’re looking to go really old school there are three works by the grand-father of American environmentalism George Perkins Marsh, including his influential text The Earth as Modified by Human Action
The cliche image is of the environmentalist sifting through page after page of old, musty, Forest Service logs, but there’s something to be said for doing all this reading and research from a personal computer. Plus, it’s nice to save a few trees in the process.