The Outstretched Shadow - Book 1 of The Obsidian Trilogy series by Mercedes Lackey. Deep in Obsidian Mountain, the Demons are waiting. Since their defeat in the last great War, they've been biding their time. Very soon now, when the Demons rise to make war, there will be no alliance between High and Wild Magic to stand against them. There only hope, Kellen Tavadon, a Wild Mage, banished from his home by his father the Arch-Mage Lycaelon Tavadon of Armathelieh.
"In alternating chapters, eighteen-year-old Darcy Patel navigates the New York City publishing world and Lizzie, the heroine of Darcy's novel, slips into the "Afterworld" to survive a terrorist attack and becomes a spirit guide, as both face many challenges and both fall in love."