Best Served Cold by Joe Abercrombie Review
As for a standalone, I certainly think this can be read separately from the main First Law series.
The same is often said for books like The Heroes (which I have read) and Red Country (which I haven’t).
The Heroes, for me, felt like a book that would be enjoyed more having read the First Law first (which I did.) While I agree it can be read by itself, I’d easily recommend Best Served Cold as a great place to start for those looking for a standalone.