Here are five places where you can download ebooks for free legally-
Project Gutenberg
Project has thousands of ebooks including popular classics that you can read download to your eReader for free. You can visit their Top 100 eBooks downloaded page to see the most popular titles of the day, week or month and the most popular authors. Once you’ve found a book you want, clicking on it will take you to the download page and you can choose from different formates such as ePub, Kindle and Txt files.
Amazon Free eBooks
Amazon has daily deals where for a short time selected titles are available for 0$. Of course, you can only read these books on the Kindle app.
Kobo Free eBooks
Similar to Amazon, Kobo too has daily promotions where some of their ebooks are available for free downloads. This one too you need to be able to install the Kobo app to be able to read these ebooks. So it’s best if you have an eReader with an android OS.
Bookbub
Bookbun includes free and discounted titles (mostly fictional) from stores such as Amazon, Barnes & Noble, Kobo, Google Play and Apple Store.
Free Booksy
Similar to Bookbub above.