I have to admit to not reading the other books but plan on buying Ninja shortly.
Just my $.02...
Edit: Forgot to directly, answer one of your questions...OOJ does come with exercises you can do at the end of the chapters that can tune your skills. And if you REAAAAAAAAALY want to hone your skills, take Rebecca Murphey's JS Assessment test at https://github.com/rmurphey/js-assessment. No kidding on this one.