Covert Threat by Amy McKinley is the fifth book in the Gray Ghost series.
This was the perfect book for me. It kept me guessing and revising what I thought was going to happen next and who was responsible for all of the weird stuff happening to Jules. I had a partially correct guess about a third of the way through but I whiffed on a few important details and I was totally okay with that.
I wish I could find someone like Trev to save me the way he saves Jules. Holy cow! But seriously, he is hot, a good guy and willing to anything to protect Jules.
If your looking for a book that will keep you guessing during the twists then I highly recommend this book.