So this book popped up on the recommended for me list on the site I use to check out kindle books. I saw the title and author and realized I must have inadvertently skipped a book in this series. Not the case. It is just a sort of companion story, a break off, let me explain some more background to you, let me sell more books, book. You don't need to read it to understand the series. However, if you choose to read the series, I think it fits right in between Passion and Rapture (it fits more, actually, mid-Passion.)
Just a couple quick love stories of some of the characters, but really not important to the series. Like I said, it really was probably a quick, oh your like the series, you'll want to read this. Sort of like The Short Second Life of Bree Tanner. This book takes the characters of the Fallen series and involves them in a side story.
A super, super quick read.
Enjoy should you choose.