All the Worlds of Men: The Histories of Earth, Book 3 (Unabridged‪)‬

If only the dangers of light travel could be contained within one world. It was easier and safer that way, when our families and the ones that we love were not at risk. 

But after nearly a year's time, back on Earth, these dangers are beginning again, with a deadly malice. “They” are after Timothy and the rest of the group of light travelers: an unknown enemy who could be anyone or everyone, and all that Timothy has to go on is a note given to him by a mysterious vanishing boy. 

It reads: 

“You are not safe.

Meet me tonight.

Stay away from the museum.”