Gallagher Girls Series

The Gallagher Girls series contains six books titled I'd Tell You I Love You, But Then I'd Have to Kill You, Cross My Heart and Hope to Spy, Don't Judge a Girl by her Cover, Only the Good Spy Young, Out of Sight, Out of Time, and United We Spy. The author is Ally Carter, and she published from 2006 to 2013. The series follows girls at a secret spy academy and their lives/loves.