Repetitive, slow, and boring.
The obvious rip-off of Peaky Blinders' characters (Tommy, Arthur, and Polly) was really disappointing for me. I thought Stirling was better than pulling a cheap stunt like that, and I'm frankly surprised that he hasn't been sued by Steven Knight for copyright infringement.
Also, it's like he co-wrote the novel with RuPaul and Gordon Ramsay, because you can't get through a chapter without at least one (often more than one) painful, paragraphs-long digression into what someone was wearing and how it was designed and the colour and cut and texture of the materials (ad nauseum) OR the same thing for the food being served to and eaten by the characters (Oh, the scallops in a fine cream sauce, with canapes, caviar on bread twists, and a '17 chardonnay, with creme brule and sorbet in five rare flavours to follow, blah blah, etc.!) to the point where it kept stalling plot advancement and really did feel like he was just using it for filler as if he was being paid by the the word or something.
Speaking of filler...similarly, the pages of domestic bliss and banal verbal repartee with the supporting characters felt like a Bronte novel. The endless flirting with Ciara and hungry descriptions of her fulsome Irish figure quickly got old. The constant reminders that the Taguchi sisters are nearly-but-not-quite twins was irritating (if you're going to keep harping on about how alike they are, use it in the plot for some skull-duggery, or shut up about it...the reader doesn't need to be reminded EVERY time they enter the scene), and the cutesy "Awww" moments when da wittle baby girls all troop in to be ever so adorable happened so frequently and to no discernible purpose that eventually you just want to V-gas the lot of them. What does *any* of this have to do with advancing the plot of the novel?
Next, the protagonist Luz has gotten so omniscient and omni-talented that she's become a caricature of the super spy (she speaks all languages and knows all varieties of fighting forms, is skilled with any weapon she lays her hands on, knows everyone who's anyone, and has a grasp of all of the history and politics of every nation and culture in the world to the point where everyone is fooled by her suave handling of every situation and there's never even the slightest doubt that she will succeed in her mission -- yawn!). She actually takes her aforementioned cutesy moppets on a mission into harm's way, but there's never really any concern for their welfare on the part of the reader because, well, Luz, right? What spy would *actually* do that, or be allowed to do that? Toss in that a good portion of her interior monologue is in spanish, which means that if you don't speak spanish, you either miss what seems intended as pithy observations, or you constantly have to pause to open Google translate, and the character soon becomes boring.
Finally, after all of the above Macguffins, the plot jumps around and Luz draws conclusions out of thin air and suddenly miraculously knows where to go to confront the bad guys...how, exactly??
I'd enjoyed this series up until now, but I don't think I'll waste my time and money on any further installments. This was just bad, lazy writing.