First of all, to those that may not know. Jeff Mariotte's book may cover various plot points and character developments up to the end of Season 5, but it's not an episode guide. Like the show, some parts of the book are graphic. In particular, Chapter 15 which focused on "blood suckers and flesh eaters" was really really gruesome.
The author is clearly a fan of the show, and I think that shows in the writing. It never felt like a dry lesson in criminology/criminal justice when I was reading the book. :)
I liked how Jeff Mariotte was able to blend examples of (in)famous cases to what viewers see in "Criminal Minds", giving us an idea of what real life profilers' jobs are like and just what has inspired some of the more interesting episodes of the show. I'd say if you're a "Criminal Minds" fanatic, you really should have this in your hands. Authorized companions to TV shows don't get much better than this.