Have you tried your local library? I download books to my Nook from my local system. The problem is that some publishers won't allow their books to be loaned electronically.
Copyrights are another problem. Many books are kept under copyright long after the initial 17 year period has elapsed. e.g. Ian Fleming has been dead for 48 years, yet the James Bond books are still copyrighted.
One of the biggest sources of free downloads is gutenberg.org. But, most of their content is limited to 19th century and early 20th century publications in the public domain because of copyrights. If, however, you like classics such as Charles Dickens, Mark Twain, Alexandre Dumas, etc., you can find them there. (As teenager, I loved the swashbuckling pirate novels of Rafael Sabatini, Captain Blood, The Sea Hawk and am tempted to read them again to see if the are still as exciting to me as the were then.) They even have Horatio Alger's novels. Their selection includes fiction and non-fiction books. They also have books in dozens of languages from Afrikaans to Yiddish.
If anyone else has any suggestions, I'd like to know. My local library is starting to build a collection of ebooks but the selection of authors is limited.