Richard Williams has just released an iPad version of The Animator’s Survival Kit. Needless to say, it is well-done. And interactive. The animation examples can be played realtime or advanced 1 frame at a time, with or without onionskinning.
The price is $34.99, which isn’t bad, but you can also download a free version that gives you a taste of what the full app looks like.
https://itunes.apple.com/us/app/the-animators-survival-kit/id627438690?mt=8