I picked this up in the library and decided to read it through. I have been using WordPress for some years, and helped others to set themselves up with it. I have also been asked to help solve issues with a number of people whose WordPress sites have been hacked and have noticed a recurrent theme with installing plugins and propensity for getting hacked. The last few years I have largely stopped using WordPress.
I read this book both as a refresher and to see if it had any insights for me about WordPress security that would allow for more stable platforms. As a refresher it was fine, but on that second point, the answer was "not really".
This is a good book for anyone wanting to get going with WordPress, and not too happy with just making it up as you go along, or following your nose! It will tell you how to do WordPress right. It was not, however, much help to any advanced users wanting to harden their systems against attack.
Personally I think there is a fundamental issue with the WordPress plugins that renders them vulnerable in too many cases. Still, it is undoubtedly good and useful software for blogging and small web sites.