FBReader
FBReader is a pretty simple reading app, but it’s also really effective. Instead of having a huge library of books, the app offers different sources where you can download free and paid books.
If you already have books on your iPhone, you can use this app as just an e-book reader. FBReader supports different e-book formats, including Epub, Mobi, and FB2. You can read pretty much any book or file you have without worrying about monthly subscriptions.
Plus, this app also offers cloud sync, so you can start reading on your computer and finish reading on your iPhone without any hassle.