Leah and Bran's relationship has been touched on in past books but the origins of it have remained murky. It is cleared up a little bit in this story when Leah's past comes to haunt her present.
The book opens with Leah, Rachel, Sage, and Anna on a girls' outing some time before the beginning of the last book. They are having some drinks (although alcohol doesn't affect werewolves) when Sage gets Leah to open up a little bit about how the two became mated. And even just this section proves Briggs' mastery of timelines as she deftly bobs and weaves between the past and present of two overlapping series.
This new story sends Charles and Anna along with the berserker, Tag, to the California mountains where a whole town has disappeared. They were there one week after being checked on by the Forest Service and then... gone. An old evil has popped up again and is joining forces with a group that has had past interactions with both the Alpha and the Omega as well as the Columbia pack.
I wish we had seen a little bit more with Bran and Leah. After the big set up of learning more about her past, we don't see a lot of them. Of course, the series does focus more on Anna and Charles so that makes some sense.
Opinions are my own