Book 3 is called 'The Circle Of Fire'. It is released 4 August 2011.
Heres the synopsis:
Now that Lia has found the missing pages of the Librum Maleficii et
Disordinae, or the Book of Chaos, the end of the Prophecy is in sight.
While Lia tries to save James from Alice's sinister grasp, she unlocks
the prophecy's final mystery and a caveat that may put an end to any
hope of ending the prophecy; to close the Gate to the Beast forever, Lia
must convene in an ancient ritualistic ceremony with all four keys at
Avebury. But that's not all. Alice, as Guardian, must be there as well.