I already read the first Mistborn trilogy back in 2017. Happily, Tor ebook club recently gave away the next three books in the series for free, probably as a bit of promotion since another Mistborn book is scheduled to come out this November.
The Alloy of Law is the first book of the so-called Wax and Wayne subseries for Mistborn, set some 300 years after the end of the first trilogy. The world and society of Scadrial is now in a kind of western / steampunk era, and the book opens with our protagonist hunting down outlaws in the fringe regions outside of the city. Tragedy befalls him in the introduction and he finds himself returning to the main city of Elendel (presumably named after Elend from the previous trilogy) to deal with the noble families and eventually investigating a series of mysterious train robberies and hostage-takings.
(I do find mildly cheesy the protagonist's first name "Waxillium" which conveniently fits the pun Wax and Wayne).
It's a very short book. The first chapters are mostly set-up and took me a few days to get past them, but once the action started to pick up, things proceed quickly and I finished the remaining 70% of the book in a couple of days. The story feels a lot less epic in scale compared to The Final Empire, if we were to compare first trilogy books. (The Wikipedia entry tells me Alloy of Law is less than half the word count of Final Empire.)It's basically a mystery investigation interspersed with large-scale gunslinging action scenes, although the ending implies a bigger picture perhaps to be covered in later installments. A brisk and fun, if short Mistborn installment.