As titles go, The Alloy of Law is a pretty bad one. It's not nearly as bad as All You Need Is Kill, but it doesn't suggest anything about the story, even if you're familiar with the magic system in the Mistborn series, and it's hardly evocative. The magic is based on using metals, so alloy is at least relevant, but you're still not getting a sense of the story inside through the title. I think it's a sign that Sanderson had hit his popularity stride by then, and realized he could name the book Three Banana Wind and still have people read the book.
The good thing is that the title of this book is the worst thing about it. Following the disappointment of the first Mistborn trilogy, I was pleased to find a readable, engaging, and intriguing story that reminded me of how much fun it was to read The Rithmatist. The theme, tone, and weight of the novel was so different from that of the first three books; Sanderson populated the novel with characters with whom we could sympathize, and they all felt more realized. Neither did he waste time with world-building, instead getting right into the story, and once it was underway, it flew by. With the first three books, I was more interested in finishing the books so I could move on to the next one, but with The Alloy of Law, I was looking for spare moments so I could keep reading. There was more of a sense of humor in this novel, too, which went a long way toward increasing the fun factor of the novel.
The story here is set 300 years after the events that concluded in The Hero of Ages, and is a mix of a western and a superhero comic book. The two main characters, Wax and Wayne (OK, maybe the title is the second worst thing about this book), are lawmen in the Roughs, which is basically the Mistborn version of the Wild West. Allomancy still exists, so the heroes have extraordinary powers, but society and its people have developed a good bit since the days of Mistborn and ash clouds. But when Wax and Wayne (groan) get involved with a string of robberies and kidnappings, of course they get involved.
The Alloy of Law is a self-contained story, and could possibly be read by itself without having read the preceding trilogy, but there are enough bits of information that would be opaque for not knowing the whole history. Also, while this novel is self-contained, it does set up the potential for a greater story arc, which will be the heart of the next trilogy in the series. And I have to give Sanderson the proper credit; he tells a great story with twists and turns without cheating the reader, and also leaves enough parts of the story dangling to leave us ready for the next book. I just hope that when he gets into the next trilogy, he focuses more on how he wrote this novel, and less on how he wrote the preceding trilogy.