Cassandra Clare published on her website teaser for City of Lost Souls and spoilers for City of Fallen Angels. She does that on a regulary basis, so all fans should stay tuned. :)
And the limited edition letter from Jace to Clary that was included in some copies of City of Fallen Angels is now on her website, too. While you can no longer buy the actual letter the image of it, handwriting and everything is now up.
Those informations were taken from her blog and you can visit her website here.