Things I liked:
1. It's like a TMI spin-off featuring Simon (who has fast become one of my favorite characters in the series).
2. Isabelle/Simon. hohohohoh way to go, Clare, pairing off the two remaining characters without a partner, but oh well, you did it well, and this is my favorite ship so far.
3. Clary is slowly becoming a take-control over girl, and not merely the damsel in distress. Quick thinking about ruining Jace's rune, but that wasn't enough. With a character like Clary, though, it's a big improvement
Things I didn't like:
1. Jace and Clary's love story has become even more angst-ridden. I liked the incest-related angst, but seriously, I could slap Jace for looking for more excuses to be unhappy.
2. Seriously, after the fight ended with Lilith, why oh why did they just leave Jace alone with Sebastian???? You would think that after all they've been through (the Valentine episode and the more recent episode with Lilith), they'd be more paranoid, and effin make sure Sebastian was secured, if they can't kill him. But, no, they left Jace, who's still pretty unstable, alone with Sebastian. It's like watching the girl in a slasher film walk around the house in the dark, knowing there's a killer around there somewhere. Sheesh.
This was a pretty enjoyable read, mostly because of the Simon parts. But I think making a book after the trilogy ended was unnecessary. Yes, I really think it was unnecessary. I liked the trilogy but enough is enough.