Dawn of Ash was suppose to come out this summer, but plans changed. Also, this will not be the final book. There will be Dawn of Ash, and another one.
Rebecca made a YouTube video explaining everything. Here it is:
Here is her Facebook Page: https://www.facebook.com/rebeccaethington.author
And her website: http://www.rebeccaethington.com/
An older video about Burnt Devotion and Dawn of Ash: