The Secret Circle: The Captive by L.J. Smith

Adam and Cassie have sworn not to love each other for the sake of their beloved leader Diana. But Faye has caught them in their one embrace. Now she uses it to blackmail Cassie, forcing Cassie to lie to Diana and to dig up the strange crystal skull that gives them all such dangerous visions. Meanwhile, students are being murdered, and the witches are under suspicion. And Faye wants from Cassie the ultimate betrayal: her vote for Faye as coven leader.

I must warn you before you start this book, make sure you have both parts
if you purchase it in it's ominbus form! Half of this book is in the first
ominbus and the second half (you guessed it), is in the second. I had a hard time waiting about a week for the second book to read! It is really good book and shows what lengths Cassie will go to to protect her best friend and the boy she loves.