Shadowhunters and Downworlders, edited by Clare (who provides an introduction to the book and to each piece), is a collection of YA authors writing about the series and its world.
Authors Who Contributed:
Holly Black / Kendare Blake / Gwenda Bond / Sarah Rees Brennan / Rachel Caine / Sarah Cross / Kami Garcia / Michelle Hodkin / Kelly Link / Kate Milford / Diana Peterfreund / Sara Ryan / Scott Tracey / Robin Wasserman
This book is a series of essays from different authors. Each section talks about different aspects of the Mortal Instruments series as well as most of the main characters specifically.
It's going to appeal to different readers in different ways. For example, I particularly liked the sections regarding the personal characters stories. How they interact with each other and learning more about each of them and their growth throughout the series so far. My particular favorite thing about this book was learning more about Valentine and Magnus. Sure, everything I already knew about them from reading the series, but this was like getting an added bonus to a more philosophical side of them and their contribution to the world.
Some of the stories/essays didn't really hit a spot with me, but everything about the Mortal Instruments series is touched on.
It's a great book for fans of the series, to those that are interested in the series but don't want to read the books (but..um WHY NOT?!)
Yes there are spoilers for those that have not read up to The City of Fallen Angels, but mostly for those that have not read The City of Bones (the first book)
Several revelations are reveled about the characters and how they are romantically. Also the settings and world building.
Cassandra Clare opens up with a very detailed and information section answering almost every question a fan might have of how the idea came about, her love for her characters and this world.