The Shadowhunter's Codex - Cassandra Clare, Joshua Lewis
If you want some backstory on the Shadowhunters, this is the book you'll find it in. I liked the way they set it up as a manual. Of course, the side notes from Clary, Simon, and Jace were the best parts, in my opinion. Not that I didn't like reading about the history of the Mortal Instruments and all that other stuff (because I did), but even fictional manuals tend to get a little boring. The notes helped to keep my interest.