I didn't like this one as much as the first book. I felt like there was too much going on and the plot was confusing.
I'm not versed in comic books, so I had to learn not only about certain comic book characters, but also various iterations of those same characters, the difference between periods in comic book history, and learn about some of the authors who may or may not have crossed over into the world Jace is currently in.
Then there were people who had appeared in the comic book in the world Jace is in which meant keeping their real names and characters straight along with their various powers and items.
On top of that there's the introduction of an Illuminati like secret society.
I'm still not sure I understand the order of events in the crime Jace had to solve. At one point she offers a theory which requires time travel and Cassius tells her that the time sword thing doesn't allow time travel. But I'm almost sure the final explanation involved some kind of time travel. (Time travel usually makes my head hurt, YMMV).
I don't understand why putting the sword through Jace's hand made her hand phase out of time but made Peter disappear and be replaced with a version of him that had made a different choice. Was it intent? Where the sword struck? And why did Tair show up before Peter was actually stabbed but long after that decision was actually made? It's possible Tair showed up at the time the decision was made but I didn't get that sense. I thought it was more recent that than because Dr. Pete had been with the NSA for a number of years. I'm going to miss Dr. Pete.
I'm not sure why Jace can't make more gunpowder or have it made. Surely they have the technology to figure out the ingredients and the percentages in the forensics lab. And they understand combustion. Can't Jace just get through the spell in the tech's head and then put him on the project?
It seems to me that without firearms the humans never really had a chance. As I understand it, not all humans can use magic and physically they can't hope to beat pires, weres, or lems. With firearms if they could get the distance to use them they might have a chance.
I still like the characters and the world I just think this particular plot was too convoluted.