I haven't read any of the other books in this series, which ended up being a mistake.
There are midquel books out there that stand on their own just fine, but Between the Cracks is definitely meant for people who already know the series.
While there's enough information thrown around through the course of the short story, none of the characters really get an introduction, and the story starts running right out the gate.
I actually didn't know what exactly was going on until I read the excerpt from the second book, which explains who the hell Tenley is, why she's suddenly not talking to her boyfriend, and why she ran off to her jerk of a brother-in-law.
Long story short: If you're just picking this up because it's free, you're probably not going to get much out of this.