Diane Duane 
Wizards at War [EPUB ebook] 

Young wizards reunite to stop a powerful enemy from robbing the world of its magic as this YA fantasy adventure series continues.Returning from their wizardly holiday, Nita Callaghan and Kit Rodriguez look forward to getting back to their normal routine. But there’s trouble brewing. A strange darkness is befalling older wizards across the world, stealing away their power. Soon, young wizards from Earth and beyond must join forces to stop an unimaginable invasion-and defend the very essence of wizardry itself.To stop the evil afoot, they must first find a secret weapon promised to them by the Powers That Be. With their alien teammates, Nita, Kit, and Dairine are on a desperate search through worlds known and unknown, racing against the minions of the sinister Lone Power who are determined to find it first. And that’s only the beginning, because for the first time in millenia, the wizards are about to go to war.