Year One (Chronicles of The One 01) by Nora Roberts

In her newest work, the stunning beginning to an epic hardcover trilogy, #1 New York Times bestselling author Nora Roberts weaves an enthralling saga of suspense, survival, and the journey that will unite a desperate group of people to fight the battle of their lives…
The sickness came on suddenly, and spread quickly. The fear spread even faster. Within weeks, everything people counted on began to fail them. The electrical grid sputtered; law and government collapsed—and more than half of the world’s population was decimated.
And as the power of science and technology receded, magic rose up in its place. Some of it is good, like the witchcraft worked by Lana Bingham, practicing in the loft apartment she share with her lover, Max. Some of it is unimaginably evil, and it can lurk anywhere, around a corner, in fetid tunnels beneath the river—or in the ones you know and love the most. As word spreads that neither the immune nor the gifted are safe from the authorities who patrol the ravaged streets, Lana and Max make their way out of a wrecked New York City. At the same time, other travelers are heading west too, into a new frontier.
In a world of survivors where every stranger encountered could be either a savage or a savior, none of them knows exactly where they are heading, or why. But a purpose awaits them that will shape their lives and the lives of all those who remain.
The end has come. The beginning comes next.