This is a prequel novella to the Night Angel trilogy, though I feel it should be left until you've finished the original trilogy.

This novella is all about providing context about Durzo Blint, one of the main characters of the trilogy and my personal favorite in the whole series. The context provided ends up being a large spoiler for the first book and contains some spoilers for the finale, which is why I wouldn't recommended reading it first. 

The book opens up with Durzo's first murder for hire, before introducing us to some other major characters readers will be familiar with. 
Similar to the actual trilogy, it's a fast paced assassin story with interesting character moments. It helps the reader understand not only Durzo, but the world and politics better.  

It's a short one, taking me around a little over an hour to read, and I would say those who loved the original trilogy should see this one as a must read.