I'm a super fan of The Mortal Instruments and Infernal Devices so it's no surprise that I got this book the second it came out. But it took me a while to read, just because it wasn't a priority. It's not another story, but rather a companion book that helps show the depth of the world.
I enjoyed this a bunch, but I found it hard to give an actual star rating on it. Mostly, it's informative, meant for new shadowhunters or those who are about to become shadowhunters. So there are a few times where I was a bit lost. It's can get very dry, too. But it really is interesting, and raises many new questions. Like really detailed, only mentioned, questions that have actually nothing real to do with the series. But I still plan on writing them all done and trying to find a way to ask the author (because I'm a huge geek and I need to know everything!).
So I really recommend this for two reasons- it looks absolutely stunning on the shelf, and if you want to really know more about the world of Shadowhunters. It's perfect for fans, but not something someone who hasn't read and enjoyed the series would enjoy.