I accidentally got stuck with nothing to read shortly after I set Fire Study aside, and as a result I picked it back up. One sitting had me read far enough that my favorite character in the series was introduced, so I actually ended up finishing the book, figuring that it didn't really cost me anything just to git 'er dun.
I don't really have anything positive to add - the plot didn't pick up, the story didn't become more compelling, and I continued to not give a damn about what was going on - but I did finally manage to place my finger on why the book was so bland and empty compared to Poison Study.
Maria V. Snyder doesn't re-introduce her characters, nor does she really do much to support them or change them once they have been introduced.
Take Valek, one of the major protagonists. I know what he's like from the first book, and I know why he's awesome. But when he showed up in this book, the author did NOTHING to describe him. You got no physical description. You got next to no backstory. You might be able to imply “boy he sure is sneaky!” from the way he showed up, but that was about it. Looking back, I started to realize that was the case for ALL of the characters. If they’ve shown up in a previous book, they get absolutely no loving attention to their personalities as far as memory-refresh or further development. Additionally, they don’t change AT ALL. The one guy’s a dick. The other guy’s a joker. The other one is mysterious. I don’t think she even bothers to use different adjectives to describe them. It’s really bad.
So, there’s my observation. I feel vindicated for finally being able to point concretely at what was pissing me off. Woo.