He's a Dead Ringer for his brother

 I'm a day late writing this, but...

Sanctuary by Mercedes Lackey is the third book in the Dragon Jousters series (and reads like the last book in a trilogy. Amusingly, there's a 4th book, which is up when I finish my library reserves.) 

Again, we're following Kiron and friends in the desert city aptly named Sanctuary as they deal with the aftermath of leaving Alta, following finding out the Mages have been using dark aligned magic to keep from aging on both themselves and the royal family. About a third of the way through, as refugees from Tia come pouring in, we find that a schism among the mages has lead to the Tian royals to be similarly corrupted. 

We really don't see much of Tia, though, as Kiron and his wing are busy dealing with another city unburied in the sand, a former wild dragon of Tia roosting there, and then the whole saving the Winged Ones from Alta and eventually the remains of the Healers. 

It's a crowded book. 

By the end, two kingdoms have more or less fallen (in Alta's case, literally, as the earthquakes caused by the Mage's big magnifying glass cause most of the city to sink into the swamp) and Kiron and friends are looking forward to a normal life. Eventually. 

I've enjoyed this reread, but I find it really doesn't have as much emotional depth as some of her other works. On the other hand, it hold attention quite well, so I can't complain.