I've read and enjoyed the first two books in Martha Well's series of planetary romances about the Three Worlds. (You can check out my comments on The Cloud Roads and The Serpent Sea here and here.) The tell the story of Moon, a member of a shape-shifting race known as the Raksura, though he's not fully aware of this at first. In fact, his origins remain obscure in both the first and second books. We learn that he's a feral soliatary, a wanderer who lives at times with the groundlings, who always expel him when they learn what he truly is.
In The Siren Depths we finally learn the truth of Moon's origins when another court (Opal Night) of the Raksura claims him from the court that has adopted him. No longer a solitary whom no one wanted, Moon becomes wanted by two groups, though he's unsure about both of them. He finds out how he came to be a solitary, and he learns some other things about his beginnings. But there's more to the story. Moon isn't sure he's getting the whole truth from the court of Opal Night, and he's right. There are deeper and darker truths lurking in the siren depths, truths about not only Moon but about the Fell, the race that preys on the Raksura. And about something more.
Wells does outstanding world building, and the Three Worlds setting is as vivid and as real as the house next door in spite of its strangeness. The characters are real, too, and a pleasure to read about. There's adventure here, with battles in the air and in the depths, and there's a thoughtful approach to the problems that the characters encounter. This may or may not be the final book in the series, but if it is, it's a fine way to conclude the story of Moon and the Raksura. I recommend that you start at the beginning of the saga, if you haven't already, and read all three books for a gratifying escape to a world of wonders.