The Alloy of Law uses the same setting as the Mistborn trilogy, but is the start of a new series, not a sequel. You don't need to have read the original trilogy to understand and enjoy this one, although there are references to the original series characters. Mostly as myths/saints/gods. It lends a certain humor value to reading the book.
It's set three hundred years later, and while the first trilogy had something of a Rennaissance-fantasy feel, this one is more of a turn-of-the-twentieth-century. The parallels between real world and fictional actually feel much stronger in this book: there's the Wild West analogue, and then there's the turn-of-the-century London analogue, where electric lights and automobiles are just coming into use. I'm glad to see the society and technology progress in a reasonable way, but a bit sorry that it doesn't feel more mixed-up instead of paralleling development in the Western world. The developments in the use of magic were more interesting to me, although less dramatic for the world as a whole. Still, it's good to see magic as a thing that evolves and changes too, in ways that make sense.
Early in the story, one of the protagonists meets his prospective bride, along with her father and her shy, pretty cousin. It's a very formal marriage-of-convenience affair: the protagonist has a title and holdings but no money, the bride's family is wealthy but has no reputation or social standing. The fiancèe-to-be is extremely business-like: she has drawn up a 20-page agreement detailing the nature of their engagement and future relationship.
The scene has the feel of a set piece, designed to make the shy pretty cousin look like the book's female romantic lead and the prospective bride to look like bridezilla.
I instantly fell in love with bridezilla. I mean, she was charting out the details of her future husband's mistresses and planning her eventual affair (after the birth of the requisite 2.5 children) with the coachman. And being quite upfront and straightforward about the whole things. It was ADORABLE.
At this point, I was really hoping this scene would not be what it looked like. To avoid spoilers, I won't say how it turned out, apart from 'somewhat more complex than initially implied'.
Anyway, the main plot is a mystery, laid out and mostly-solved in an entertaining fashion. Brandon is clearly planning a sequel -- important points are left unresolved -- but it's a satisfying book on its own. I give it an 8.