City of Glass
This was my favorite of the Mortal Instruments series. The main plot revolved around an upcoming battle between Valentine and the Nephilim, with the Downworlders holding much of the power to either side’s success. It felt like all of the buildup of the first two books was finally going somewhere interesting. Although there was still the awkward brother-and-sister-but-not-acting-or-feeling-like-it dilemma, it was resolved in exactly the way I expected it to be (took them long enough to figure it out!) and the other minor storylines were tied up quite nicely.
I felt like many of the characters really grew into themselves throughout this book. Clary establishes her place in the Nephilim world (and fortunately the author doesn’t do the typical cop-out of somehow making her main character a fabulous warrior with absolutely no training or prior experience); Jace discovers and comes to terms with who he really is; Simon also finds a place in this new world that was opened up to him; and we finally get a glimpse into who Clary’s mother really is.
Overall, I’d recommend to anyone reading the first book or two to finish off the trilogy — this book was by far the most interesting of the three.