I can see why this guide is ‘unauthorised’. When it comes to being a guide to Twilight, it isn't really a guide at all.
From the very first page this book is riddled with errors regarding the Twilight Saga. It seems doubtful if Lois H. Gresh has even read the books.Even getting the order of the books in the series correct was a difficulty! I spent most of the time I was reading it wondering how it managed to get published with so many mistakes.
All this book really amounts to an overview of vampire myths with a few quizzes trying ride on the success of Twilight by slapping it into the title.
But if you're addicted to anything vampire and want a light read, by all means buy the book. It does provide some interesting points about Twilight in relation to vampire myths on the whole,but don't let the title mislead you into thinking ‘The Twilight Companion’ is something it is clearly not.