This is an excellent book, being both comprehensive and well written. Sadly it suffers from the Achilles heel of many technical books: the product it describes has changed radically since it was written. E.g. the example application, based on Grails 1.x, needs major surgery to work on 2.x, and the referenced version of Spring Security is obsolete.
That said, it is still a decent introduction to an amazing framework. To get the best from it, I recommend using Grails 1.1.2 while working through the example application, then installing the latest version (currently 2.2.1) and reworking the code until it works again. You will learn (and swear) a lot.