1) What level of programming knowledge do you assume the reader already has?
I'd expect even a true beginner to be able to pick up the book and get something from it. Here's the problem, here's the solution. This book is more lab than lecture, so you'll want to look towards another book like Programming Groovy for deeper explanations of the subtleties.
2) Do I need to know Java to follow your book?
It'd be helpful. Groovy augments Java, so having some understanding of the Java ecosystem would be a good thing to have. I've never talked to anyone who's come to Java via Groovy -- I'd love to hear their perspective.
3) What is the scope of the book? In other words, how "deeply" does this book teach Groovy to me?
This book wasn't meant to be a definitive guide to Groovy. For that, I'd look to Groovy In Action. I think that my book hits all of the high points of Groovy, but it is by no means exhaustive.