This 4th book in the Retrievers series isn't terrible but it isn't very interesting, either. Mostly, it seems to serve as a bridge (no pun intended) between the previous book and the next.
As other reviewers have commented, the main reason for the serviceable read is due to the concentration of the story arc on the politics among the "major" players in the series, namely the Fatae, Lonejacks, Cosa, and Silence, and the increasing tension due to the escalation of attacks by Nulls against anyone who has, or who appears to have, magic. To be fair, this political situation does have a key role to play in the plot as it also drives what's happening; the problem is that rather than making the information succinct, the various unproductive political meetings take up too much of the story. The various factions don't get very far in their dealings, and neither do we, as the reader.
The situation between Wren and Sergei, played up on in the book's synopsis, doesn't really have much twists and turns as was hinted, until the end of the book, just like the intriguing prologue. A hint of a connection to the prologue appears but it's so subtle that it might as well not be there. In any case, by that point, one has either forgotten about the prologue or one has torn one's hair out wondering what role it plays in the story.
In essence, there's a lot of exposition but not much action.