If you are registered with Amazon, you can find some FREE Kindle eBooks simply by visiting the official Amazon Kindle Store from your Kindle e-Reader.
Simply select 'Best Sellers' and then select 'Top 100 Free'. The book/s can then be quickly downloaded onto your Kindle via WiFi.
You can now also email EPUB books to your Kindle and it will automatically be converted to the best Kindle format available.
Useful links:
Project Gutenberg (Free eBooks)
Project Gutenberg Mobile - optimised for small screen e-Reader viewing
e-Books Apps and sites:
Libby: Use Libby to read books from your public library on a tablet or phone, simply by registering using your library card. You can either download the Libby App on your tablet/phone or else you can use the web browser version.
Sooperbooks: Read or Listen to short stories for kids on https://sooperbooks.com/