Demonglass is the second book in the Hex Hall series. Another cliff hanger, only this one is really hanging. I have to get that third book!
Sophie is spending the summer in England with her father, who's the head of the Prodigium council. They're living outside London at the Prodigium council headquarters in Thorne Abbey. Sophie is in for a surprise when she finds out who else is staying at Thorne Abbey, and once again her life is in danger. This time from a direction she never expected. Very highly recommended.