I always enjoy Frances Hardinge novels. This one is set in a world where everyone lives in an underground city with a well defined class system where the underclasses are exploited by the elite classes. In this world, people do not have the innate ability to form facial expressions and must learn a handful of ‘faces’ to use when they deem them appropriate. A child is found and adopted by a poor cheesemaker and she has no memory of her life before, but he makes her always cover her face. She believes it is because she it too ugly or disfigured (they have no mirrors) but discovers eventually that is it because her face constantly changes and reveals her thoughts (ie she had what we would consider a normal face).
This is a young adult novel, and the feisty female protagonist (the girl with the face like glass) must overcome trials and perils to discover her identity and right (at least some of) the wrongs of the worlds she finds herself in.