PLOS stands for the Public Library of Science, and this is an open access source (free to all via the Internet) containing full text of journal articles on all areas of science and medicine.
The following journals make up the PLOS platform:
PLOS One: this online journal has full text original research articles in all areas of science
Oxford Handbooks Online contains various books in the Oxford Handbooks Online. The tool provides access to individual articles as well as access to the book. Make sure you "narrow your choices by availability" and choose unlocked and free.
In the search box, use the phrase environmental sustainability.