Well, this one kind of shit the bed. So much so that I'm doubtful I'll bother with book 4 when it comes out later this year.
There were a lot of problems, but the chief one is that Cora suddenly took a turn towards TSTL, and I've finally come to the conclusion that she talks like a badass but is actually ultimately useless.
We didn't get to see her in a lot of action in the first two books - she didn't really handle any of the problems - but I was tolerant because she had to keep her magical heritage secret. In this book, she finally ditched the secret and fully engaged...and got her ass handed to her repeatedly. She was a stupid, pathetic chump. In this book, we see her get played like a fiddle, act utterly brainless about obvious things (raise your hand if you didn't realize those cards combined to be a summoning thing. No one? No one? Right, so we all saw that was super obvious except Cora), and just let herself get tortured nearly to death. Twice she had the bad guys in her hand, and twice she let them go. She is a world-class dumbass in this book, and needs to get sit in the corner with a blankie while the adults handle the problem, because she is clearly incapable.
Speaking of dumb shit, evil grandma has figured out how to summon Cora - she summons her and binds her, to torture her in an effort to steal Cora's power. She's rescued ... and this is time number two in the book where the chumps that are Team Good Guy just let the leaders of Team Bad Guy go. But like, what's the solve to the summoning issue? Why didn't evil grandma summon her again as soon as she got to a new lair? That's right, there's no reason. All this effort to rescue Cora the Useless after she's summoned, only to let the summoner go so they can all do this all over again soon. Best plotline ever!
The other major problem is that grandma, who was always a bit sketch but in a weird gray way - she was a bitch, but she has stepped in to help them several times, even risking her life to save her daughter in the last book - suddenly morphs into a full-on cartoon Doctor Evil-style villain. I mean, it's fucking ridiculous, complete with evil glares galore, glee while torturing family, etc. No gray area here, Doctor Evil Grandma literally, at one point, drags her bound and gagged daughter to the front lines of a fight and threatens to have her daughter (Cora's aunt) raped by her whole army unless Cora hands over Macguffin Evil Book, and allows Team Evil to leave. Cora does this immediately, so it's an all-around win for Cartoonishly Evil Grandma.
Even more amazing is that Cora bends over backwards in this book to keep people from going after psycho bitch granny. Cora knows her grandma is a mass murderer pretty early in this book, but she hides that info as much as possible, and even makes a deal with Lucifer to keep him from going after grandma when grandma starts massacring his people. Why? No reason. Just Cora being pathetic in literally every possible way.
It's really, really bad. I mean, two stars is being generous.