"Hidden" begins five minutes after "Destined". The book starts from Lenobia's point of view.
P.C Cast said the Marked red fledgling Shaylin newcomer will have a larger role in "Hidden". Just the engagement between Neferet and Darkness (white bull). Kalona will have problems with his new job as the Master Sword of House of Night.
It is expected that meets new characters.
~ From the HoN Wikia
The release of "Hidden" is scheduled for October 16, 2012.