"The Final Key" is book #2 in a series that precludes "Primary Inversion", the book that first introduced Sauscony Valdoria, heir to the Skolian throne. The first book in this series was "Schism".
You see, there are essentially 3 civilizations in Asaro's future universe. The Allied Worlds are those established by humans from Earth. The Skolian Empire and the Eubians (the Trader Empire) are remnants of another human civilization (the Raylicon Empire) that left Earth long ago and established a presence in the stars, while the rest of the Earth-bound humans lost this part of their own history. When the Allied humans finally left Earth to explore the universe, they were suprised to find that humans were already there.
Allied humans are much like you and me. Skolians, however, are generally empaths, and their civilization is based on a royal family with exceptionally strong empathetic abilities. They are "psions", who can read emotion almost as well as communicating with words. The Eubians (or Traders) literally derive pleasure from the pain of a psion, who transmits the emotion and amplifies it like an antenna. Some of the Eubians are pretty nasty in their taste for the pain of their Skolian pleasure slaves. A few Eubians, however, aren't so bad.
The Skolian Empire and the Eubian Concord have a nasty history with one another. The caste-like system of the Trader Empire rests on a slave/indentured servant relationship between upper and lower classes. Any psion captured by the traders is immediately enslaved and sold for the purpose of being tortured in order to pleasure his or her captors. Such a culture, of course, is repulsive to the Skolians, and therefore the two cultures don't get along.
How unfortunate it was for humans from the Allied Worlds, full of expectation and curiosity, when they stumbled out to the stars only to find that someone had beaten them there. To top it off, these two advanced civilzations had been at each other's throats for millenia. The Allied humans assumed a role of neutrality, and have stayed that way, even though they do not necessarily condone the Eubian slave-trade.
I became addicted to Asaro's universe when I first read "Primary Inversion", which began at a time long after Soz had graduated from the Academy. "Schism" and "The Final Key" preclude "Primary Inversion" and tell the story of Sauscony Valdoria (or "Soz") when she was training to become a Jag pilot. Jag pilots are an elite fighter group composed entirely of highly empathetic psions. Soz can deal out some serious butt-whoopin' when she needs to, but her personality is more of an INFP for those of you familiar with the Myers-Briggss personality test. Soz is an introverted, intuitive, feeling and perceiving person who is exceptionally intelligent. She is not all touchy-feely, however, as her calculating mind also makes her a great soldier. Thus her personality makes for great character development.
Asaro first wrote a series of exciting books including "Primary Inversion" and "The Radiant Seas" (all about Soz), "Catch the Lighting" (background on Soz's brother Althor), "The Last Hawk" and "Ascendant Sun" (introduction to Soz's other brother Kelric), and "The Moon's Shadow" (about Soz's son) which all take place in and around the Eubian-Skolian war. These books are brilliant and they have led to stories that still need to be completed. These books are some of Asaro's best and much of her writing revolves around these stories.
"Schism" and "The Final Key" are closely related to the above-mentioned books, but they fill in historical background to those books. Asaro has dedicated a lot of time to creating this universe and filling in background information, where books like "Skyfall", "The Quantum Rose", and "Spherical Harmonic" tell stories of other members of the Valdoria family. The Valdorias are a fascinating bunch of people, and Asaro has woven their family history into a very solid series of novels. I have found her map of the Valdoria family tree, conveniently located in the back of each of her books, to be very valuable in sorting out all of the characters. Overall, the background provided in "Schism" and "The Final Key" is invaluable as it provides support for her intricate story network.
In "The Final Key" much of the story is told from Soz's perspective when she is a young adolescent woman coming to terms with her own identity and sexuality. This book finally brings the reader to the point where the Skolian Empire goes to war against the Eubian Concord. The reader finally knows how Soz was born, where her father came from, how her brothers and sisters fit into the story, and how the war started. The reader watches Soz grow from a girl into a woman, and finally Soz has a chance to experience combat.
Books like "Primary Inversion" and "Ascendant Sun" give us some detail about the war and add to further development of Soz and her brothers Althor, Kelric, and Kurj (just a few out of many). There are a few cliff-hanging books at the end of Asaro's timeline that leave her fans wondering what comes next... last I read, Kelric had suffered from some memory loss and was stranded on some backwater planet, Soz got married (a great story how that happened), had some kids, and now her son has achieved a position of royalty, but not like one might expect.
Asaro has created a unique balance between hard sci-fi and genuine story-telling. Many of her novels have a romantic side to them that is sometimes cheesy, probably causing more than one reader to raise an eyebrow, though she is never intrusive with her ideas on love and relationships. Her stories are definitely first rate and many of her books are among the best science fiction written in recent years, combining the elements of science fiction (complex technology, limit-pushing physics, interesting ideas about biological evolution), with stories about real characters with emotion, feeling, and intelligence.
In addition, many of her books could stand alone. In a bin full of Asaro's books, a reader can grab a text at random and begin reading. A logical place to start might be with "Schism", followed by "The Final Key". One might wait for another part of the Triad series, or just jump to "Primary Inversion" and go from there. In any case, all of the Skolian saga books are worth reading.