Hooray for Better World Books! I have a copy of "Deep Ecology" by Bill Devall and George Sessions (1985). This was an early work that outlines the Deep Ecology platform.
@Roots isn't too focused on philosophy and theory, but any discussion of the modern environmental movement has to address the "how" and "for what" of the struggle. I've wanted to read Deep Ecology because it's supposed to be focused on the first part: the ontology of environmentalism.