This was a thoroughly disappointing and generally terrible book.
I like the series, and when I saw that the story's main character was Mo, the normal main character's (Bob's) wife, who often sallies forth at the end of the books to save Bob with her magical violin, I was optimistic.
However, it was not to be. First of all, the dynamic between Bob and Mo is that they have different personalities, and Mo especially has a job that strains her to the max, and their differences are what help keep her grounded in her high stress supernatural secret agent assassin job. However, in this book, Mo was written as a female Bob, with the same ADHD frenetic energy and drive.
But, and this is the really bad part, the author's method of turning a character female is to make her flaky in relationships, really focus in there about her makeup regimen and how many sets of clothes she has, cheat on her husband, and generally be terrible at everything all the time. She lets herself get mansplained so many times, at the tail end they even gave it to her as a freaking superpower.
As for the plot, it was also terrible. It starts off by rehashing the end of the last book, then telling us that, "Oh yeah, didn't we mention? There are superheroes now." Superpowered people are cropping up because the stars are aligning just right (blah blah, this used to be a mixture of computer stuff and Lovecraft mythos, but I guess we're taking a break). So, people are getting superpowers, and they're taking the form they think they should have based on public perception, which in this case means the Marvel comic movies , and they want Mo to create and take charge of a new unit to deal with supervillians, by creating super hero teams to A: combat the villains directly, B: get the more powerful supers under direct government control and C: create a good role model for emerging supers.
Even before Mo is roped into this, a supervillain emerges. It turns out to be... not a supervillain exactly, but a rogue group of cops! They had a scheme to use Mo's violin to mind rape Britain into following their orders, not realizing that, aside from being an evil act on its own, would kill everyone by leaving them open to alien entities called 'feeders' that would eat their brains out, a side effect of magic in this universe that they would be opened to. At this point, Mo's violin tried to take over the world, and the violin was tackled away from her.
It's a real problem that this book can be summed up as 'A bunch of bad stuff happens to Mo, and she's too weak to take care of herself'. Come on! She's the one who takes care of business!
Problems:
1: Mo is written as a hyperactive autistic dishrag. She lets men in her life jerk her around, and they don't get their comeuppance, either.
2: Mo is weak. She has zero control over her violin, and despite a line in there stating that she was well on her way to being able to 'take care of herself' without her magic violin, we've never, ever, seen any evidence of this, nor do we here. Prove it, Stross.
3: Mo wonders how her people are going to overcome having their brains continually being eaten by the Feeders, but never does anything about it.
4: Mo is upgraded to Senior Auditor at the end for no good reason. The whole point of the Senior Auditors is that they're all super sorcerers, and she has no arcane powers of her own anymore. And if she did, she'd get her brain eaten by the feeders. In fact, she's vulnerable to that already because of her 'being ignored' superpower.
5: The author wrote up the sexual harassment and blatant misogyny that Mo experienced as a god damned superpower.
6: The entire book was just a list of exceedingly boring meetings, one after another... This was seriously 95% of the book! The few times there were action, it was usually not Mo doing anything.
7: Even a random shmo could make a better superhero team than Mo did. She just sat on her butt for months! Then Mo goes along with it when they decide to make the 'superheroes' into regular police somehow. Lame!
8: So, they're trying to find this specific supervillain... and one day they discover that an entire subway station has been sealed off by perfectly placed stainless steel! Look, I realize that the book took a different tack at the end, but at this point everyone should have been all "Holy Bollocks, this is the work of our supervillain! We're breaking through this instant! Officer Friendly, punch the hell out of this thing!" Instead they leave, and a couple of weeks later they're told it was just some missing paperwork. Weeks! What the hell were they doing all that time?
9: We never find out the full story about what happens at the end. I guess some people died, but some people got the feeders in them, but lived? The heck? These damned things are written as if they're action reports, so there's no reason not to have that in there.
10: After an entire book of Mo slowly writing off her marriage to Bob and getting more romantic with the superpowered, influential, well off, and charismatic Jim, she's just told to 'work things out with Bob'.
11: Hey, Bob died for real when he the whole "Eater of Souls" thing happened... and might still be dead? Are they even really married anymore?
12: Are they pretending that the violin is gone for good, when anyone with the right computer program can pop open a portal to that same world and grab it again?
13: Stross is really playing fast and loose with his mythos. Before, we got alternate worlds destroyed by CASE NIGHTMARE GREEN, or where the nazis took over and lasered Hitler's face in the moon, or what not. This time, the King in Yellow had a city on earth like Atlantis, but it was destroyed, but also somehow existed in an alternate place with two weird moons, but also also somehow created music in our world and later got someone to make evil violins?
14: Oh yeah, they only ever found one real supervillain! He could effect people's minds, and they had him in their office. They had him beat, then let him go. That was super, super dumb.
15: And another thing! At one point, as a red herring, the creepy Senior Auditor asked Mo if she could be sure that Officer Friendly was really Jim. He was worried that it could just be random people in super power armor, which evidently is being made with alien tech by multiple arms dealers. Mo worried about this for months! It's stupid because earlier in this same book she showed us that her violin is a freaking tricorder that can track life signs and read souls! How about... just use that?
So TL;DR... The usual main character, Bob, usually has adventures, and gets saved by Mo at the end. But, Bob has gotten incrementally more powerful with each book. So, to keep the same dynamic, the author had to power up Mo too. Unfortunately, he built her up to be the leader of a super hero team, then yanked it away at the last minute and made her a powerless 'auditor' instead. In so doing, he damseled her, and subjected her to misogyny and sexual harassment, and she just had to take it. Screw you Stross!
But why? My theory is that when he was planning Mo's powerup, he wanted it to be similar to Bob's. Bob got a physical powerup, but also made a big deal about getting into the management track at work. He not only replaced his boss Angleton as 'Eater of Souls', he got his pay grade too. Stross started this book by hamfistedly breaking Mo's cover and getting her into a position where she immediately got a triple promotion. Then, after trying to write his equivalent of putting together the Justice League, but accidentally writing hundreds of pages of boring meetings, infidelity, dream rape, and lame britishisms, he cut his losses, made it all about the evil violin at the last minute, threw in a new mystical baddie, and completely ruined Mo (took away her useful magic, gave her a superpower she can't turn off that'll eat her brain)... then gave her another promotion anyway.
A better version would have been to make the damned team in the first week, actually seen some of these thousands of other supers at some point, done something with that, then at the end - she uses her super mystical music powers to dominate her violin and turn it good, or at least bend it to her will for good. Or make a new violin that isn't evil. That would have been much better.
I'm sorry Mo. You're very poorly written.
Update: The next book in the series also follows a different main character, this time one of the random vampires from a couple of books ago...