Me and the reviewer weelenny "Dranoel" disagree wildly in regards to another product of the Hollows (the Graphic Novel), but in this case, I have to actually completely go with his grade. This is, definitely, a 5 star product. Not only it's exquisitely made in heavy stock paper, with awesome printing, but it's shock full of information.
While there is SOME stuff that came from the Kim Harrison site, it's just a few bits and bobs that actually fit here, and are totally worth to see again in print. However, it's NOT like the case of the very ghastly (and thicker) Illustrated Guide to the Twilight Series, which is nothing but a reprint of the TwiLexicon information, padded out with an interview with SMeyers and really unnecessary information (cars? really?). By comparison, The Hollows Insider gets it ALL right: new knowledge about the world, about the characters and organizations, about people never before mentioned (but that had to exist somewhere, right?). My only (minor) complaint is that it's oddly sized -- but as I said, a minor niggle, and never in any way a deal breaker.
The strong point of the book was exactly the new information and knowledge about the series -- we get not only great chunks of the bible of the series, but there is information nowhere else to be found. Plus, the care to make this an assembly of intel made by a reporter accidentally hexed by main character Rachel, and justify it with a ton of tiny little details (love the expense report) tells a tiny story on itself.
This is very clearly a labor of love, made not to earn a quick buck like some other franchises (coughTwilightcough), and it involved clearly a lot of work; other authors who consider publishing guides to their world should take a good look at this book first and then learn from it.
A definite 5 stars.