To Die For by David Baldacci

Homeland Security's Travis Devine gets a strange assignment--to escort a 12-year-old orphan named Betsy to meet with her uncle, Danny Glass, who is under investigation for RICO charges in Seattle. Betsy is a handful but eventually bonds with Travis when he promises to find out who killed her parents. And Travis finds out there is way more going on behind the scenes with Uncle Danny than he realized. And of course, Travis is still dealing with the mysterious Girl From the Train, who already tried to kill him once and doesn't seem to be planning to stop trying. All in all, it's quite the page-turner.