Meet Heckerty is a delightful story app that's great for students with learning difficulties. I love the way that in the 'Read to me' option the words are highlighted as they are read aloud, the ability to tap a word at any time and have it read out to you and the very easy navigation. (There is an IOS app of this one too)
Neon Tiki Tribe book apps are great fun, probably best for students up to the age of 11, older kids would find them too young I think. They focus on issues such as bullying and use a dyslexia friendly font , the accompanying song soon loses its charm but maybe not for younger readers! (+ IOS)
Moon+ Reader is good for reading ebooks, the scrolling line under text (you can change the speed of this) is very useful and there are a number of other adjustment you can make in the app such as changing the background theme, brightness etc.
Just a few but I will add more if I find them.