Fourth Day by Zoe Sharp

In this eighth installment of the Charlie Fox thrillers, Charlie volunteers to infiltrate a cult.  In a short time, she finds she is drawn to the organization's leader and feels he is helping her with her challenges.  Of course, her boyfriend Sean can't understand her choice and feels she is brainwashed.  As usual in the Fox novels, there is a great deal of action, and the story ends in a cliffhanger.  Also as usual for these books, it is well written with an involved story and a well-developed main character.  It is longer than some of the earlier books in the series, but is still worth the time.