A fair number of these collections are free from their originators all year long. These include U.S. and other government documents (NASA, Dept. of Energy, FAA, UN, etc.), Project Gutenberg, Renascence Editions, World Public Library, and a variety of international collections. If you want to see the breadth of ebooks available these days (and download a few for your reading/listening pleasure) now is the time to do it.
Copyright restrictions do apply. Be sure to check with the originating group to see if the book will be available later on if any of these look good as textbooks, supplemental reading, or source materials.