In this sizzling conclusion to a mordant fantasy trilogy, magic is more curse than blessing for 15-year-old Reason. She could wield it, and die young like her forebears; or refuse it, and go insane like her mother. A solution seemed to appear when a distant ancestor transferred his unfathomable power to Reason and her unborn child. But her best friend Jay-Tee is still dying from magical burnout; her baby’s father has rejected her; her mother has been kidnapped; her evil grandfather is on the prowl; and now a social worker is investigating. No wonder Reason retreats more and more into the cool comfort of pure magic and abstract mathematics. Can she withdraw there, and still stay human? Alternating chapters by Reason, Jay-Tee and their friend Tom recount this crackling blend of fantastic adventure and soap-opera angst with vivid splashes of Aussie and American slang. A wry concluding twist mitigates a too-pat conclusion, and adolescent readers will be left pondering their own hard choices. Not a stand-alone story, but the entire trilogy is a worthwhile purchase. (Fantasy. YA)