The main arc plot was an interesting idea. Dante and Jeph are forced into making a deal with the devil. They'll hunt 4 top flight demons and otherwise be hunters for him for seven years, during which time Dante will have his protection. Japh will get his full demon powers back and they'll have access to the devil's intelligence network.
The problem was that all of that is vastly overshadowed by the relationship between Dante and Japh and I found that relationship highly problematic, enough so that if it isn't better by the next book I think I'm done with the series.
Japh treats Dante like a child, then acts angry when she acts like one.
He's changed her on a genetic level without her informed consent and still refuses to tell her what exactly that means. He doesn't stop her from looking, but he doesn't help her search either (and I suspect the missing oragami notes contain things that are important so I wouldn't be surprised if he's actually hindering her search). That's bad enough. What's worse is that he treats her like a slave "for her own protection."
He doesn't tell her anything because he "doesn't want to worry her" even though not having information has led to her nearly getting killed on multiple occasions. At one point he uses his greater strength against her, lifting her against the wall so high her feet can't touch the ground and terrifying her.
It's abundantly clear that he doesn't think of her (or treat her) as an equal or even someone he respects. He humors her in a very condescending manner. He keeps talking about how she needs to just trust him, but he shows absolutely no trust in her. He doesn't trust her to take care of herself, he doesn't trust her to make her own mistakes and find her own solutions... he's nauseatingly paternalistic and I fail to see why she still loves him.
Apparently he's sworn to protect her because of the relationship between them, but it's clear that means only physical protection. He doesn't seem to care if by physically protecting her he destroys her emotionally and psychologically and kills everything that make her who she is.
For a while Dante seems to get that, but when she learns he didn't take the devil up on his offer to kill her to regain what he was suddenly all is forgiven.
It seems to me that those are two very separate issues and I was more than a little frustrated and angry that Dante conflated them.
What Japh is doing isn't love.
They talked about it a little but it's too soon to say if it makes a difference or even if Japh gets the problem. Trust is a two way street that has to be earned. The book seemed to end with the idea that since he never agreed to kill her for the devil and has saved her life on numerous occasions, somehow it's all okay when it's clearly not. You can't keep a person wrapped in cotton all her life, it's not fair and eventually she'll suffocate.