The last of the short stories in the Seventeen series, it focuses on how the two main characters in the fourth novel, Ashstorm meet. And it is rather embarrassing as they happen to be robbing the same bank and the same time. It shall be clear, this is how partnerships start.
It adds some new information, but it is not essential for the main series. I, myself, however always really want to complete series, so I'm glad I've read these as well.