The Son (sic) Also Rises
Both Clary and Valentine suffer major setbacks at the end of City of Ashes while Jace and Simon come out ahead.
First, Valentine's evil plot, while not necessarily thwarted, is somewhat diminished when Clary destroys his boat with her super Rune. Unfortunately, we have no idea where he's run off to, and he still has the Mortal Cup and Soul-Sword, so he'll probably come back stronger than ever, like a cockroach that's been stepped on.
Clary, who has been unable to make a decision between Simon and Jace the whole book, ends up with neither. Simon tells her he just wants to be friends because, "When I'm with you, I want to know I'm with the real you, the real Clary" (Epilogue.49). We wish he'd tell her who that is, though.
And Jace is suddenly respectful, saying he wasn't right to push Clary into a relationship when she didn't want it. Bad timing, Jace, because now Clary is pretty keen on the whole Cersei/Jaime dynamic she initially refused.
However, it's not all bad. Jace gets reaccepted to the Institute and feels like he has a family again, and Simon, who has died twice by this point, is reborn as some sort of magical sun-immune vampire after drinking Jace's blood. That's a gamechanger.
Speaking of gamechangers, on the very last page, a mysterious stranger tells Clary she can wake up her mother. That's the perfect cliffhanger setting us up for the next book.