This second installment of the Mortal Instruments series (there's going to be six books, apparently? Yay!) picks up right where City of Bones left off.
(And now, spoilers for CoB:)
Clary (the red-headed girl on the cover) is still very much attracted to Jace, and Jace to her - but now that they've found out that they're siblings (and that their father's the evil antagonist Valentine), things get awkward. Personally, I like that the author's dealing with that can of worms, instead of making her characters magically not attracted to each other anymore once they learn that they're related (I'm looking at you, Mercedes Lackey and James Mallory.).
Also, Clary and Simon's relationship goes to the next level (Level 1 was "they're friends", level 2 was "they're the best friends" (see City of Bones), and Level 3 is "kissing time!"). Also, Simon gets an upgrade - I won't spoil it for you, but let's just say that there's plenty of foreshadowing.
Plus, Clary's mother is in the same condition as she was when City of Bones ended.
Oh, and the war that pits our antagonist Valentine against, well, everyone is going swimmingly. It looms ever closer, and to be honest I'm really looking forward to reading the third installment of this series - if the next book's climax/final battle/chaosfest is to top this one, it should be pretty epic.