Beka is one of George's ancestresses, in training in this book in the Provost's Guard in Corus. She grew up in the Lower City and has voluntarily returned there for her training, and quickly commits herself and all her new friends (other members of the Guard, other trainees, old friends from her past, random women in the area she coerces, a Lady Knight and various rushers at the Rogue) to solving not one but two major criminal rings. One is killing the laborers hired to mine fire opal in cellars in the Lower City. Another is kidnapping and sometimes killing babies and small children in an effort to get their adult family members to cough up items of minor (and eventually major -- the fire opals) value.
Pierce's novels have always benefited from interesting side characters. I think this one has the strongest supporting cast, however. It's _very_ easy to imagine what Lady Sabine, Rosto, Kora, Aniki, Tunstall, Lord Gershom, Berryman and others are getting up to when she's not involved in Beka's activities. Writing it as Beka's journal (with the prefatory matter being other journals and letters) is a very different -- and very succesful -- point of view for Pierce. I hope she continues with it. And of course it's good to see Cat again.
There was one minor boneheaded error (a crib?! a crib?! Come on!) and one major failing of world building. Where on earth do these people piss and shit? We know where they clean up; we even know that some of them clean their teeth. So I'm betting Pierce has a theory about where they eliminate. But when Beka (who supposedly raised her younger sibs to a large degree, which does make sense) has the young Ashmillers visiting, the youngest is wearing diapers. No. First, the kid is too old (walking) to be in diapers (for this time frame). Second, there's no way they could _afford_ diapers. Third, while diaper changes are depicted (with a _toy_? The Ashmillers have a _toy_?), diaper emptying/cleaning is not, which is also deeply flawed. My guess is, babies/children that age just squatted in the gutter, and when they screwed up and did their business indoors, their caretakers kicked the shit outside and then kicked the kid (possibly in reverse order) to make sure they didn't do that again soon/often.