Absolutely great fantasy. It begins with people randomly turning into birds, and the search for these people leads to an underground where the lost and unwanted of Moscow live - both the politically dispossessed and mythological and fictional characters. As the story goes on you get many little stories about why the people ended up there.
It's similar to Gaiman's Neverwhere, in broad strokes, but this story has a lot more to it. It's more straightforward (in both style and content) too.