Houghton Mifflin Harcourt is an amazing resource for books in the classroom. It comes with free books or you can purchase leveled stories within the app. You get about 20 books for $5. The students can read them or listen to them. The bar at the top lets you quickly go to the level you want. When you purchase a book it's automatically placed under the correct reading level. I love it!
Any book you purchase for your kindle can be downloaded through the cloud to your iPad.
I have an app but I think you need a subscription to Storia through scholastic now.
You can purchase leveled books for $1 on your iPad through the app store. I organized them by level on my iPads.
Search language technologies, inc. or LAZ Readers.
EPIC! is a Netflix style app for books on the iPad.
It's $5.00 a month for unlimited books for any age.
With the iBook app you can purchase any book and read it online instantly.
Here are some reading passage apps I use in my classroom: