Having utterly and totally loved Diving Into the Wreck, I was thrilled to discover that it was not, as I had erroneously thought, a standalone novel, but rather part of a series. What I loved so much about DItW was the pitch-perfect sense of isolation, claustrophobia, and foreboding that characterized the sequences in which the characters explore long-derelict spacecraft with pernicious, incomprehensible, often fatal technology on board. Those parts of the book scared the hell out of me and gave me nightmares. However, it turned out that this element was largely absent in this, the second book. That's not to say that this represents a flaw in the book. City of Ruins is a solid continuation of DItW - it's just that it doesn't include much of my favorite part of the preceding book.
My main criticism is that there were numerous occasions when I had to reread certain passages several times because I didn't understand, usually in spatial terms, what was happening. For instance, the first time the characters enter a cave beneath the titular city, I couldn't figure out whether they were dropping into a vertical hole in the ground, entering some sort of mineshaft, or what. I still enjoyed the book a lot, but this was a recurrent issue that frustrated me to a not insignificant degree. Also, there are a lot of minor characters that I found difficult to distinguish from one another (I had to list them on 3x5 cards like when I was reading Kushiel's Dart), but the protagonist feels like a much fuller character, and overall, story is more of a first contact sort of thing anyway, so that's not a big issue.
And speaking of that first contact thing, I'll keep things spoiler-free, but the book handles it very adroitly in terms of pacing and tension. I wanted to go downstairs for a snack during one long reading session, but I kept saying to myself, "Well, one more chapter, then I'll go." I feel like I've talked a lot so far about what I didn't like, but the book really is compelling and vivid. I enjoyed it and recommend it.
P.S. If someone wrote an essay comparing and contrasting the underground/street level elements of this book with the same elements in The Dark Knight Rises, I'd totally read it. The book's prompted a lot of not yet fully formed thoughts about the underground as the subconscious, with a seemingly incomprehensible presence slowly, methodically being understood. I know that's super vague and probably makes no sense if you haven't read the book, but I don't want to spoil anything.