The Amazon Kindle app is one of the most popular reading apps. Like most other reading apps, it lets you keep bookmarks, tracks your reading progress, and lets you adjust font size and screen brightness.
This reader app platform is nearly identical to Amazon Kindle in features and selection. Amazon tends to be a little cheaper, and Google Play Books does not currently have a subscription service like Amazon Unlimited.
What you get with Apple Books is very similar to what you get with Amazon Kindle and Google Play Books. One big difference many report is a smoother user experience with Apple Books.