What is the solution in the book Pretty Little Liars?

In the book series "Pretty Little Liars" by Sara Shepard, the identity of "A" is revealed to be CeCe Drake, who has been tormenting the main characters—Aria, Emily, Hanna, and Spencer—throughout the series. CeCe's motivations stem from her complicated history with the Liars and her obsession with Alison DiLaurentis. The series concludes with various twists and reveals, exploring themes of friendship, betrayal, and the consequences of secrets.