Even Better Sequel: according to a poll on the Gallagher Girl wiki (taken when there were only five books), the fifth book is by far the voters' favorite book. The second one is the runner-up, followed by the fourth and third. The first book is the voters' least favorite.
Harsher in Hindsight: The climax of CMH becomes this when it's revealed that Dr. Steve, the guy who was pretending to steal the Gallagher Academy alumni list, is really an agent of the Circle who tried to make Cammie kill herself.
Les Yay: Cammie really likes to point out how hot her roommates are.
"She (Macey) was running toward me. So strong. So fast. So beautiful."
Nightmare Fuel: Cammie gets brainwashed and forgets an entire summer. Later, the same person hypnotises her and orders her to kill herself. If she hadn't been stopped, she would have done it.
In the sixth book: the song that Zach remembers his mother singing to him when he was a child. Hearing Catherine sing the song herself after she's captured is rather chilling.
Above the plains up on the hill there stood a castle bold. A gleaming palace made of white, a pillar to behold. The horsemen lived in service to the castle and the crown. But the knights rose up and killed the kings. And it all burned down.
The Circle of Cavan's master plan: Liz's hypothetical plan to start World War Three. Poor Liz is horrified when she finds out that they're actually using it.