I have been making use of Amazon Kindle App and ComiXology for a while now. They are both very good when it comes to the books that you can find on them. There's this app that I used to read novels online but I can't remember the name again.
Kindle is a good reading app. I have read a lot of books on it, and that is something that I am always super excited about. Getting to read your favourite books on those apps.
I don't think I've used any reading app's on my phone. I've been afraid that I'd use up too much of the battery on my phone and my phone would die right when I needed it.