It is a great book. Cassandra Clare is an amazing author with the ability to draw in reader's. The characters in this series are mesmerizing. I actually read these, (in fact, I couldn't put them down), and I am in love with them. They are for a slightly more mature reader. I would feel comfortable letting my own kids read them at any age they wanted to. However, I know some parents would be concerned about some of the situations that arise in the book. There are allusions to sex, some heavy romantic scenes, and references to drug use (not actual drugs, but "fairy drugs"). There is never anything that is actually explicitly stated in the book; however, kids can get a pretty idea of what Clare is alluding to. As an educator, I do not have a problem letting my 7th graders read these. They see and hear far worse on television and radio. I am more cautious with my 6th graders. I make sure to let them know what they will encounter. Most are voracious readers, and used to much more pg-13 scenes than are presented in Clare's work. I have requested letters of parent approval for a few students just to be on the safe side. The main character, Clary Fray, is a great role model for young teen girls. These books remind me of a cross between Twilight and Harry Potter.
The bottom line: a great read for kid's who need a book (series, she's written almost 10 books now!) to love. Kid's will become absorbed in these. I am far more concerned with that than worrying about them encountering a PG-13 scene.