I thought I could give a reasonably brief synopsis for the book but couldn't come up with anything. Rather, I will dive into what I liked about the series.
For those that know me it shouldn't come as a surprise that I really enjoyed this book. Sanderson is hands down my favorite author. The Cosmere and all of his fantasy books are just so much fun to read. Warbreaker was one of his older books (it came out in 2009) and it sort of shows. I read this after finishing Oathbringer and was surprised to see a character in this book that also appears in Oathbringer.
I love Sanderson's books mainly because of his interesting use of magic systems. Warbreaker introduces a concept of BioChromatic Breaths. The idea is that every person born on this planet is born with 1 breath. Humans can use these breaths to animate objects to obey simple commands. For example, you can animate a piece of rope to grab things. Breaths used to awaken objects can be retrieved, but if you use breathes to take something inanimate and make it alive those breathes can't be returned. People can give their breathes away also which leads to some people achieving interesting powers with the more breaths you have.
This was by far one of the more unique magic systems I have seen in a Sanderson book. It was a bit more vague than his other books but that isn't a bad thing. I found the idea interesting and I'm a bit sad we didn't get to see more interesting uses out of it. I loved Mistborn since the characters would often do some really incredible things with the limitations in the magic system.
My favorite part of this book though has to be th eintroduction to Nightblood. This character was very entertaining in Oathbringer and I was surprised that it makes its first appearance in this book.