It’s not intended as a language tutorial (after all, it is still being debated), this is often repeated; however, you’ll find some pieces of code: this is real thing, not just a dream. It would be hard to explain isolates without showing some code. I would say the main goal is to demonstrate Dart is not just some funny thing that won’t survive any longer, that it deserves some interest from Web developers. And it does it well!
You want to read it? This ebook is freely available through O’Reilly: http://shop.oreilly.com/product/0636920025887.do.