I'm a huge fan of the Beyond series by Kit Rocha. It has great world building, with a fun group of loyal characters. This book, is part of a larger Alpha After Dark anthology. This book can be read as a stand alone. This story features two characters which we haven't see much of in this series.
If you haven't tried this series yet, this is great sneak peek into the world of Sector Four and O'Kanes. You will also learn some about Sector Two. I highly recommend you give this series a try if you haven't yet.