Unfortunately, the "middle child" syndrome continues its time-honored tradition here. Although there is some action, for the most part "Ruby Knight" is a place-keeper, marking the time between when Sparhawk figures out what he has to do and finally does it.
At the end of "Diamond Throne", it was revealed that the only thing that would save Queen Ehlana was the Bhelliom, a legendary sapphire of enormous power. Once it was a part of the Royal Crown of Thalesia, but was lost during the monumental Zemoch War several centuries earlier. Now Sparhawk has to track it down, dig it up, learn how to control it, and get it back to Ehlana, before the spell preserving her life fails, and those of his friends who are supporting the spell die with it.
So Sparhawk and Co. trek across the continent, fighting a few battles, meeting a few characters (some of whom will become important later, so pay attention), avoiding the Bad Guys (as led by Annias, the churchman who's seized control of Ehlana's kingdom and is looking to take over the Church as well, and Martel, Sparhawk's nemesis) and learning much more than they planned about the REAL history of their world. We finally find out what the heck is the deal with Flute, the Styric orphan Sephrenia adopts, but as I said, this book mostly exists to keep Books 1 and 3 from bumping into each other.