I have now read it (sorry it took so long!) and here's my review.
It's a well-written book. The exposition is clear and the author knows the subject and what he wants to say.
I was impressed by the straightforward approach of Grok when compared to what I dimly remember of ancient Zope experiences. The idea of a modern, agile, simple framework over Zope's admittedly powerful foundation has a lot of merit.
OTOH, I am not really convinced that I should drop Django for the small web projects I have scheduled, but if you are looking for a framework, and are not heavily invested in another, Grok is certainly worth checking out.
And, if you want to learn Grok, this book does the trick.