The tone is set when Dresden attends the trial of a sixteen year old boy accused of dark magic. The kid had stumbled into mind control without knowing the laws of magic, and now the White Council can only make one choice: the penalty for breaking the laws of magic is death. As Dresden leaves the trial, the Gatekeeper gives him a cryptic warning of dark magic being used in Chicago. 
 The first few books in the series were pretty campy, trying to be Noir and coming off more like the hammy versions of the genre. The last few books, certainly since Blood Rites, have felt a little like place-holders. Very little actually happens in Blood Rites, making the book feel more like just a vehicle for the big reveal at the end. Dead Beat had a lot more going on, but still seemed to be trying to get through a load of exposition. 
 Proven Guilty had some of the same feel to it, and we learn a great deal of background about the "Dresden Pack." We also see quite a bit of pay off in Dresden's character development as he deals with his strained relationship with the Carpenters, his connection to the fallen angel Lasciel, his "will they, won't they" relationship with Murphy, his feelings about Ebenezar, and, of course, his rather difficult relationship with the White Council. 
 There were several difficult issues tackled in the book, perhaps the biggest being Molly, the Carpenters' seventeen year old, having a crush on Dresden. While I understand that it's a situation many would rather not read about, and I see several reviews calling Butcher some variation of "creep" for writing about it, I actually quite appreciated it. The fact is that this situation happens, and it happens a lot to young girls who have troubled relationships with their families (and are therefore already vulnerable in all sorts of ways). Acknowledging that the older man might be tempted, that the refusal might be difficult, just added realism to scene. Throughout, Dresden modeled the (mostly) appropriate course of action for the older man to take - he refuses, he sets explicit boundaries, and he never ever takes advantage of the situation (except, of course, for the impromptu lesson involving an ice bucket challenge). 
 I also appreciated how Dresden and Murphy handled their feelings for each other. While certainly not ending the "will they, won't they" subplot, I was pleased to see them talking out their feelings and options like mature adults. Dresden also gains a new understanding of his mentor, Ebenezar, and begins the process of repairing their relationship. All in all, Dresden grows up a lot in this book, and seems set on a good course to repair all the damage that came to the fore in Dead Beat. 
 That said, there was a little "plot critical" silliness. The events of the book circle around a horror movie convention: SplatterCon!!! Yet despite two separate incidents that, collectively, led to several deaths and hospitalizations, it's just assumed that the con will continue. Never is the possibility of cancelling the rest of the event seriously considered. I can understand continuing on after an incident that left an old man beaten up in a bathroom, but once someone dies, it almost seems in poor taste to keep on celebrating horror movies. 
 Butcher has gotten much better at setting up tricks that will come in handy later in the book. Early on in the series, Dresden would pick a few potions to make, seemingly out of a hat, only to find that they happen to be the exact potions that he needs. It was a little silly. Here, however, Little Chicago is introduced early on, but it's given a firmly plausible purpose, even if it happens to be exactly what Dresden needs later on. There's also some teasing, where Dresden thinks that he will need it, but then doesn't, then later does for a different reason.