I absolutely loved the Nightshade trilogy. I have even read the two books in the prequel series, so I was excited to hear that Andrea Cremer was writing another book set in the Nightshade world. This series is for adults and is definitely not YA due to some graphic adult content. That is the reason she decided to use a pen-name instead of the name associated with her YA books.
Captive tells the story of Shay's parents, Tristan and Sarah Doran. Tristan is a Keeper and Sarah is a Searcher, which in theory means they should be mortal enemies. What starts out as a ploy on both of their parts to gain information, and perhaps Sarah's freedom, turns into far more. Tristan and Sarah are both strong and independent characters. As the story developed I couldn't help but love them, which is probably why I was on Team Shay since they very beginning of the Nightshade series.
All of the events take place prior to Nightshade, but I would recommend reading the Nightshade series before Captive. There is a lot of history and detail behind the Witches War, Keepers, Searchers, Guardians, and the nether creatures that is discussed at length in her YA books. It is touched upon briefly in Captive, but it was described at a level I would consider to be more of a refresher. I don't think I would have appreciated this book nearly as much as I did if I didn't already have this background information.
If you are an adult fan of the Nightshade series, I would highly recommend reading Captive. It contains all of the action and suspense I have come to expect from the Nightshade books with the addition of a steamy romance. I will definitely be counting down the days until the release of the next book in this series.