Sept 2016 Update:
I no longer support or recommend this series. Book 5 has taken a very unfortunate turn towards graphic sexual content that is not appropriate for the young readers this author and publisher are targeting. Reader beware!
The Assassin and the Healer (Throne of Glass, #0.2) by Sarah J. Maas
This was more of a short story than a novella. It give more insight into Celaena and what makes her tick. Also, according to the authors blog, the events in this store wind up impacting the story later on. I can’t think of how they impacted anything yet, so I assume it’s something coming in the last book.
Also, the only way to own it is to purchase The Assassins Blade, which compiles all 5 novellas in to one book. I already own the 4 original novella separately and didn’t want to re-buy them, so I found the novella online here: http://www.movellas.com/mobile/app/read.htm?movellaId=201301311152267426
From the authors blog:
“Then there was the fifth, unexpected novella—The Assassin and the Healer. Okay, it’s actually a short story. I wrote it for the 2013 World Book Day in the UK as an exclusive bit of content, and wound up loving it so much—loving Yrene Towers so much—that I wanted to make sure that it was someday available to all TOG readers. (Especially because the events in Healer wind up having a big impact later on.) Yrene is the only character other than Celaena to get her own POV in these novellas (save for a very short scene with Arobynn in Empire). So, make of that what you will.”