The setting is the Three Worlds, inhabited by humans but also by other races that share some human characteristics but are different in some basic ways, such as having the ability to shift shapes and fly with eagles' wings. Okay, not eagles' wings; check the illustration on the cover. This race is called the Rakusa, and in the first book, Moon, a Rakusa who's been solitary from birth joins a tribe. You can click on the above link for a little more of a summary if you want one. Now he's the consort of a queen and a member of a court that's moving to a new home as a result of events in the previous book. When they arrive, they discover it's a wonderful place . . . but something's missing, something vital that has to be found.
The rest of the book is a quest, and it's a dilly. It takes us to a place even stranger than anywhere we've visited so far, and I'm not going to describe it here. You should find out for yourself. There's some excellent world-building, some fine descriptive writing, and plenty of action. The characters and relationships from the The Cloud Roads develop and deepen. If you haven't read The Cloud Roads, it's probably a good idea to read it first. Do it, because you don't want to miss this one.
The Cloud Roads is showing up on a lot of "year's best" lists, which is no surprise to me. I have a feeling we'll be seeing The Serpent Sea on them, too. Check it out. As for me, I'm ready for the next adventure.