Periphery (Periphery #1) by AA DaSilva:
 
Charlotte barely survived the accident that killed her husband four years ago. Resuscitated a savant, she struggles to find meaning in her survival.

When she meets Simon, a mysteriously familiar stranger, they are drawn to each other with undeniable magnetism. But Simon is contracted to a black-ops agency.

With the agency on her heels, and Simon claiming her heart, Charlotte’s past and future collide when she's offered a chance to reconnect with her late husband.

As secrets are revealed, motives uncovered, and alliances are formed, Charlotte must choose…between the fate of the world and the fate of her heart.