Dumah's Demons is an "accompaniment" book to She Speaks to Angels. If you're reading the series, this is a short story I would suggest reading between book 1 and book 2 (which is supposed to be coming out this month).
I really liked this story. It is sort of a backstory of Dameon, explaining the issues between him and Kian a little better, and also showing you what he's up to when he's not at school. It also introduces you to Dumah, a character that will be introduced in book 2. Basically, it answers a lot of the questions that I had while reading book 1. And she does a great job doing so.
Note: I received a copy of this book in exchange for my honest review.