This is an odd one. I'm going to give it four stars, because I do think it deserves it, but I'm going to put a huge caveat on that, which I'll get to in a minute.
Ms. Hawk certainly has a grand imagination. All her worlds, whether I can get into them or not, are well-detailed, well-thought out and the world-building is pretty smooth, giving you want you need to know when you need to know it without burying you in extraneous details. That is the same here. She's put a different sort of twist on vampires here. Certainly, vampires actually being demonic spirits isn't new, but in this world, vampires are thought to not actually exist. Ghouls and werewolves, sure. But vampires? Hah! Except they do. It's the method of transfer from one host to another that differs, and I quite thought it to be more complex and have the potential for more conflict that in typical vamp lore. We certainly get to see those conflicts emerge here and start to be explored, and since the way the possession works, you can still sympathize with Gray while understanding why Caleb is justly upset by all this. Neither of them asked for or wanted this; they're just going to have to find some way to make do. End of series spoiler:(show spoiler)
I really enjoyed all the stuff that gets explored here, though I did think Caleb's trust issues with SPECTR and with John went away or started to fade just a little too quickly. I would think Caleb would be more suspicious than he ends up being, and would therefore be looking for more ways to manipulate the situation and John - but that doesn't happen. Of course, he's forced to stick around since they're hoping John will figure out a way to get Gray out of Caleb's body, but that doesn't mean that someone like Caleb, raised with a deep distrust of the system, would thaw out as quickly as he does.
Now for my caveat - obviously, this is M/M(/U) (Caleb considers Gray to be "male" but Gray really doesn't have a gender), and romance is a must. There's no real romance in this first book, which is fine because that would've been misplaced. There's plenty of lusting and lusty thoughts, of course, and John gets a little too handsy with someone he's supposed to be protecting/holding in custody. John gets a wee bit unprofessional, but doesn't cross the line into totally unprofessional until they finally have sex in the last chapter. I have a couple of issues with this that prevented me from being able to like the scene, aside from the extreme unprofessionalism:(show spoiler)
The writing is strong enough, and the characters and premise are interesting enough, I'm willing to at least give the next book a try and see how this develops from here.