Another thriller!

This book follows a couple, going up into the mountains on a couples retreat. He has dreadful nightmares, fears that wake him in the night with violent kicking and screaming.  She has her own past, sordid and messy.

They meet with the doctor, who records their every move on video cameras. They walk in the forest, and meet a woman who suddenly vanishes. They are looked after by a cook who speaks no English and brings plates to other doors.

How does this one end?  Read it and find out.