The monergism.com website contains “a curated digital library of historic Christian theology”. The books align with the Reformed tradition (see Our Faith for a statement of beliefs of the editor and founder, John Hendryx). Currently there are over 1,300 free ebooks on the website. Find the ebooks here. The website has an explanation page for various ebook approaches here such as Apple Books and Kindle. The About Us page gives an overview of the website.

You can sign up for a periodic email newsletter from Mongergism Books here. I subscribe to this newsletter and have downloaded many good digital books from this website. Most recently I have downloaded The Everlasting Covenant by Benjamin Keach. I download the EPUB variant and then “share it” to the Kindle app on my Android phone. There is also evidently a Send to Kindle app for Android devices.