Just as a sidenote, you might not find all these apps useful. It's really up to you to find out which apps work for you, but if I have missed any, let me know!
The Kindle app almost seems like a must have. I own a Kindle and admittedly I do read the majority of ebooks on my Kindle. However, there are times where this app has proved a necessity. Situations where you can't take your Kindle but want to keep reading the book, you can download the ebook to your phone and you can stay in sync so you'll pick up right where you left off on every device.
I found out about Litsy from other book bloggers ironically on Twitter. It's essentially a social network for readers. After creating an account and following people (I'm joshandabook, hint hint), you'll get a feed of quotes, recommendations, pictures, reviews, and you get to post them too. Plus, you can update what book you're reading on your profile and keep track of them. It's now one of the apps I do go onto regularly, though I do need to post more! I think you should give it a try!
Goodreads is the app that every booklover has heard of, but we all use it for different reasons. I primarily use it to organise books that I've read and for doing the Goodreads Reading Challenge. But people post reviews on the site, some people are part of groups, it's really up to you how you want to use it if you do use it.
Of course, there are tons of apps that booklovers can use, and there's also no requirement for you to use any apps too. These are just some of the apps I use, and I have found new apps I'd like to check out too, so perhaps there will be a new list in a few weeks time!
Happy reading, and have a great Christmas!