This has been kicking around under my ownership for a while, and I decided to read it now because the title character was mentioned, in passing, in the last Patricia McKillip book I read (Od Magic). This made me go, "Huh, is Mavin a mythological character, not just a fictional character?" No. It's just an homage.
Still, it deserves an homage.
I really liked this book. It does show that it's an early work (1985, her 4th or 5th novel, I believe). This is both good and bad. The prose is less masterful and some elements of the plot are more 'typical' than in her later works. However, while the strong feminist elements that Tepper is known for are indubitably here, the tone is less didactic than in her more recent books.
Mavin is a young woman who has grown up in an isolated community of rather inbred shapeshifters. This generation, the group is experiencing a severe lack of women. When the rather naive Mavin finally realizes what this situation will mean she is expected to do when she comes of age, she decides to break with tradition and set off on her own to gain her independence and discover her abilities. Unexpectedly, she is soon caught up in an adventure involving plots, politics, plague, and a legendary race of beings called 'shadowpeople.'