This is the fourth book in the Dragon Jousters series, and there isn't a whole lot of explanation of the ongoing situation, so I wouldn't recommend trying to pick this series up in the middle. I was slightly confused about the situation myself, even though I've read all of the previous volumes, and I'm not sure if that's because I'd forgotten some stuff that happened or that Lackey skipped some events between the end of Sanctuary and the beginning of Aerie. But the situation ended up making sense.
This book had a slightly more complex plot than the earlier books in this series, and there was some problem with pacing, as the first half involved relatively minor events and the second half was overwhelmed with big cosmic stuff but still had issues in the day to day lives of the characters to cover. This volume also brings the fairly ordinary characters we've been following directly to the focus of major events for the whole civilization, raising small issues of believability, but for the most part, it works. All in all, I definitely found it a Mercedes Lackey book -- there were plenty of problems I could identify, but my overall experience was definitely positive. Lackey has the ability to make her characters come alive and matter to me even when the setting, situation, and plot don't really work.
It's also unclear from reading this volume whether the series is over. It seems quite unlikely that Lackey can top or even match the events in this story, so even though the main characters' lives are not over and would probably still be enjoyable to follow, it's not clear whether more books would really work.
The Aerie of the title is a second city unearthed from the desert sands by storms, an event that I don't really remember happening in Sanctuary (thus my confusion about the continuity of events). Kiron starts out dealing with mundane issues of settling the Jousters in to this new city because priests have taken over all the space in Sanctuary, and sorting out his relationship with Aket-ten and wrestling with the age-old problem of understanding the opposite sex. Aket-ten is serving as a courier and working to get support for having more girls trained as Jousters, and there's considerable friction over this because all Kiron can see is all the problems that will flow from men who want dragons being offended at being passed over in favor of girls, and all Aket-ten can see is the importance of giving girls a chance to prove themselves. Kiron's personal life becomes more complex as his long-lost mother turns up, and turns out to be a manipulative, stupid, self-centered bitch, but still his mother, and the plot thickens when a lone border guard from an eastern settlement is found dead in the desert. Kiron and company investigate and discover a border outpost mysteriously completely emptied of humans and also under some influence which smothers magic. Rakaten-te, the Chosen of Seft, a very reclusive and very powerful priest, becomes involved, and he, Kiron, and Aket-ten discover the secret first of the magic suppression and then the spell which caused all of the people to walk into the desert. Rakaten-te breaks the compulsion spell, attracting the attention of its captors, and after they fight off a huge wave of magically-summoned scorpions, they return to Sanctuary and receive divine guidance: the Nameless Ones are attacking, and the solution is at Aerie. Kiron understands a prophecy which is cryptic to the people less familiar with Aerie, and all of Aerie and important people from the rest of the kingdom, working together, excavate a buried temple and find six crowns with the symbols of six of the most important gods. Ari and Nofret, the King and Queen, get two of the crowns. Rakaten-te gets the one for Seft. And Kiron and Aket-ten get two. The sixth isn't claimed. The Nameless Ones arrive with an overwhelming army, held off with "Akkadian fire" dropped by dragons, and the Nameless Ones' creation, described as not yet a god but about to come one, comes to attack. The five put on the crowns and become avatars of their gods, holding off Tamat (the Nameless Ones' demigod), but they can't win the fight without one of the avatars dying so that the god riding him can manifest his full powers. Rakaten-te accepts the duty, for Seft that he serves is the god of difficult choices. Tamat and the Nameless One's army are both destroyed. Under the direction of the personally present Seft, Kaleth, who had been the Mouth of the Gods, becomes the new Chosen of Seft (for, though it is a very secretive cult, it's a very important position that can't be left vacant), and Marit, Kaleth's wife, becomes the new Mouth of the Gods. Kiron and Peri (the peasant friend of Kiron's mother that said mother expected to marry Kiron and return to being peasants on Kiron's family farm) tell Kiron's mother to get stuffed, and Kiron and Aket-ten acknowledge their love for each other and appear to have moved past their differences.
A good Mercedes Lackey read, a story with some inspiring aspects but a number of problems. I'll give it an 8 out of 10.