Ugh! NOOO! Not the "MCs refuse to talk to each other so Big Misunderstandings" occur! And not the "ex comes back and causes trouble" trope. You're better than this, Ms. Hawk. So I could knock off half a star for that nonsense, but the rest of the relationship stuff was strong enough to overcome it. (And I guess when you fall in love in just a few weeks, before you really get to know the other person, it's not too far out of the realm of possibility for these things to occur.)
Complicating things further is just the basic way that possession works in this world, and we get to "go inside" the head of another victim. It's a great way to counter what's happening with Caleb/Gray and seeing why anyone would be extremely concerned about John at this point, and why Caleb is worried about losing himself completely if the 40 days come up and they haven't succeeded in exorcizing Gray. How can he really trust what Gray tells him, that Gray won't take over his mind, especially when they're getting better at sharing his body.
And then cliffhanger! Thank goodness this series is complete already. Onto the next!