Fyre: Septimus Heap book 7 by Angie Sage
In this spellbinding finale to an epic series, Septimus Heap must decide once and for all where his heart lies - with Magyk or with Alchemie and Physik. When, at last, Marcia agrees to allow the recently re-instated Castle Alchemist, Marcellus Pye, to open the Great Chamber of Alchemie and Physik, she fears she is unleashing more than she understands. But Marcia must learn to trust Marcellus, and together they must rid the Castle of the evil Two-Faced Ring. Caught between the two, will Septimus be able to bring both sides together?