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:
And since this whole first series and part of the second series is already released, I know that Caleb and Gray aren't able to be separated, so I assume they will have to and do eventually come to some mutual understanding/acceptance of their fates.
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:
1) While they just came from a horror show of a death match with the lycanthrope and their adrenaline would be high, causing them to act rashly, I just couldn't stop thinking that Caleb's breath and mouth must've tasted like blood. Gray had just drunk a lycanthrope dry, and Caleb didn't even stop for mouthwash. Plus, even with near-instant healing powers, Caleb would've had some blood from his own injuries sustained during the fight. Shower first?
2) Holy consent issues! Caleb wants John and vice versa, but Gray was obviously way uncomfortable with all this and had no idea what was going on. Even though Gray's an unwanted hitchhiker inside Caleb, and Caleb was desperate to get laid for a variety of legit reasons, that still means that Gray hasn't given his consent to this. At this point, we don't know much about how this possession works. We know that sometimes Caleb is aware of Gray and can carry on discussions with Gray whether aloud or in their head, and sometimes Gray seems to "go away." But we don't know if Gray is "there" all the time or has the ability to "check out" or just makes it appear like he's checked out. Gray's had access to the memories of all his prior hosts, including their sex lives, but those were always in sepia tone, if you will, and this is the first time he's experiencing it firsthand and in technicolor with stereo surround sound. That he's subdued and quiet afterward probably just means he's processing and trying to make sense of what just happened, versus being traumatized by it, but I hope this is resolved before Caleb and John boink again. For now, I'm labeling it dubcon. YMMV.
3) Actually, consent is a huge issue throughout the story, what with the forced possession. Caleb doesn't want to share his body with Gray; Gray didn't intend to inhabit a body that didn't stay dead, and in fact has no control over which bodies he does inhabit. So dubcon/noncon is just a fact of this premise. Caleb's miserable, Gray's confused and doesn't know what's going on. Neither of them are really happy about this, though Caleb's pain is clearly much more prevalent and pressing. Still, it's more or less a mutually distressing experience, until the climax. Gray overrides Caleb's consent when he decides to not just kill the lycanthrope but drink all its blood. Dude! Caleb's a vegetarian! :P And also, that's gross. Gray would've even gone after Caleb's SIL if Caleb hadn't begged him not to. So maybe you don't care about #2, because Caleb's free to do what he wants with his body without having to check in with Gray - and I would agree to that IF this series wasn't sold as an M/M(/M) romance. But it is, so my issue with #2 isn't so much that it happened in this instance - because like I said, Caleb's desires were totally valid - but concern about how this is going to be handled in future installments. If you want me to believe this is a mutual romance among all three, then at some point - preferably sooner than later - the various issues of consent need to be addressed.
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.