Cassel Sharpe's world is a place of light and dark -- he was raised in a world of mobs and con-men, but he wishes he could live among the law-abiding people. 
 But he finds that those places are not as different as some people think in "Black Heart," which throws even more snarls into the teen transformation worker's life. Holly Black wraps up her Curse Workers trilogy with gritty, witty writing and intertwining subplots -- but the real draw here is the tumultuous relationship between Cassel and Lila, and what will happen to them. 
 Cass has never been further from Lila Zacharov -- not only is she furious at him, but he's working for the FBI and she for her father's mob. But he finds himself being drawn back in when he learns that his mom is in trouble -- she stole the legendary Resurrection Diamond from Zacharov years ago, and if Cass wants to ensure his mother's safety, he'll have to get it back. 
 He also gets sucked into a bizarre scheme at his upper-crust school, where a student claims she's being blackmailed. But it turns out to be something very different, pulling Cass and Sam into a clash with the faculty. 
 Finally, Agent Yulikova enlists Cass for a special mission to transform the bigoted Governor Patton. Suddenly he realizes that he's doing the same thing for the government that he would be doing for a mob family -- and the government's motives become increasingly suspect as the day approaches. The only way out of this situation is to make his own solution.. 
 The Curse Workers trilogy is one of the most original urban fantasies in years, and "Black Heart" is a thoroughly satisfying end -- we've got moral greyness, impossible choices, corrupt government agents and even shadier mobsters. And Holly Black manages to wrap things up in a plausible way, without making anything (whether it's proposition two or Cass/Lila) seem too pat and tidy. 
 Her writing here is like a dirty obsidian blade -- dark, gritty and sharp enough to cut ("Lucifer Morningstar himself could learn a thing or two from the conviction with which Barron lies"), and she slowly builds up the tension until something has to explode. And somehow, Holly Black manages to still insert some very non-cliche teen romance angst (especially when Daneca starts dating someone... unsuitable). 
 The only problem is that the whole subplot about Mina Lange feels... well, interesting, but not connected to anything else in the story. It's like a short story was inserted into the novel. 
 But hey, Cass seems to have finally reconciled himself to the life he's destined for -- being a transformation worker means he will always have people trying to use him, and that neither the mobs nor the government can be depended on. And his tempestuous relationship with Lila finally reaches the point where both of them realize what they want, and how they feel. 
 The ending isn't tidy, but neither is Cass's life. Instead, "Black Heart" is a strong, intense finale for a beautifully dark little trilogy -- and it confirms Holly Black as a truly brilliant writer.