When I started this book, I assumed it would essentially just be a review of what these books are about. What is a tree? What is an owl? What is a Ga'hoole? But it was much better than that. It goes into more depth on a lot of things that the books don't really cover, as well as telling stories from the perspective of quite a few owls that didn't have many stories written on them. In the end it breathes quite a bit more life into the books that I read prior.
I will say, it does have that one notable screw-up in lore. Lyse's sister dies from a coyote attack in this book, where as, in his own book, she dies in a fire. So that's awkward, and has likely haunted the author for quite some time. It was a bizarre choice to not only write a guidebook before the series was done, but also have another person write it for you. But it is what it is. Otherwise, it's essentially flawless if you like the series.
Reading through some of the reviews, I think a few people may have mixed up this book with Lost tales of Ga'hoole.