A Shadow of Treason — Tricia Goyer

Sophie discovers that nothing is as she first imagined. When Walt, the reporter who helped her over the border, shows up again after Guernica is bombed, Sophie is given an impossible mission. She must leave behind the man she's fallen in love with and return to the person who betrayed her.

Another layer of the war in Spain is revealed as Sophie is drawn into the international espionage schemes that could turn the tide of the war and help protect the soldiers from the International Brigade...she must find a way to get a critical peice of information to Walt in time.