The most used book app on my phone is Kindle. It has all my books from my kindle and my phone linked through it and it's how I download books I receive in emails when I want them on my phone. You can access the Kindle ebook store and download free samples, but the one drawback is that I can't (that I can discover) buy books, even free ones, from my phone. Other than that this is the easiest and best reading app. Also, free download!