BookBub isn't actually an app. It's a website that sends you emails tailored to your book selections. These emails contain new release updates from your favorite authors & free and discounted books in your favorite genres and authors. I love getting this email. I have such a large collection of ebooks ready for me to read now that I've signed up for BookBub. And it only sends me emails I want to see. No spam. Just emails that deal with the authors/genres that I chose when signing up. And don't worry, you can always go back in and add more authors afterwards if you forgot one. It offers free ebooks for Kindle, iPhone, Nook, and a few others! And, also free to sign up for! Yay for free stuff!