Midnight's Park - Callie Simmons, #1

On a beautiful October day in 2005, over two hundred thousand people vanish from the face of the earth. Daniel Brandt is skeptical of the news - until his girlfriend disappears from the passenger seat of his car. The world finds its own way to deal with the losses, and the dust of the previous weeks seems to be settling. That is, until Amalie London - a supposed employee of Royal Research Corporation - comes out of nowhere and points the finger at Daniel.
Amalie convinces Daniel to help her retrieve a quantum computer from Royal, claiming it's their only chance to bring back the "ghosters". But it turns out not to be that simple. After a series of dangerous and illegal stints, Daniel finally comes to learn the truth about the ghosters - they will have to travel back in time to bring them back. And the repercussions of time travel have consequences on his present he couldn't possibly have imagined.