This is the all-around best e-book reader app for Mac, iPad, and iPhone. – Yomu EBook Reader Review

Yomu is simple but great. It feels like a real Apple app, and not like a cross platform app that doesn't fit into the Apple ecosystem. It supports all the major formats. It syncs your books and information on your iCloud. It doesn't require you to sign in, make an account, or share your data with third parties. Awesome. Thanks to the Yomu developer!
Review by BaskerV on Yomu EBook Reader.