The Witch of Duva (The Grisha #0.5) by Leigh Bardugo is a folk tale which has nothing to do with her Grisha series. I found it rather odd, confusing in places, and overall not very interesting or entertaining. But then folk myths aren’t my kind of story.
To buy by itself on amazon.co.uk is £0.38, but strangely you can read it free in this anthology on amazon.co.uk: Here.
Glitches (The Lunar Chronicles #0.5) by Marissa Meyer is set before the events of Cinder, when Cinder was just a child moving in with her new family. It didn’t really give anything new or add anything to the story, but it was nice to read about the characters when they were younger.
This is included in the free anthology I’ve linked to above for The Witch of Duva.
Among The Nameless Stars (For Darkness Shows The Stars #0.5) by Diana Peterfreund is the prequel to For Darkness Shows The Stars. It had an interesting story in itself (I haven’t read the book so I can’t say how it incorporates/adds to the main story) and I really enjoyed reading about Kai making his way to the big city, getting in trouble, and the things setting themselves up for the main book, which I’ll definitely have to check out.
This is free on amazon.co.uk: Here.