I read the book 4 years ago. I think it was hardback. It had a white mask with red and blue trim on a black background. I believe the title was one word and started with 'Never.'
The premise is that no one can control their facial expressions. Their faces stay static until the day they die. However, some people can create facial expressions. These are traded between people and are the highest form of currency. The elite have many, while the poor might only have one or two.
Also, the book takes place underground. It's a sprawling network of giant caves and tunnels. It's later revealed that this network is sentient, at least to some degree, and gives the people magic to keep them down there. I believe this is also why people don't have facial expressions.
The main character (female, described as looking average, henceforth referred to as Never (not her name!)), for some reason, is the only person capable of controlling their facial expressions. Her adopted father and master is a master cheesemaker who makes magic cheese. Other magical things include wine, perfume, et cetera. He has a disdain for the court and refers to it as the stereotypical den of vipers. It's revealed that he used to be part of them. He makes sure Never wears a mask (the one on the cover) because she's very valuable and doesn't want people to learn about her expressions, lest she be taken away.
Early on in the book, Never (I think) runs away from home. This is where my memory gets fuzzy. At some point, she attends some sort of ball(?), where a master thief, I believe called the Klepto... something (Kleptomancer? Kleptomaniac?) shows up and steals something. Never gets taken too, or follows, or something. Point is, she goes along with him. Klepto is revealed to be... not particularly sane. I think they meet a couple more times later on.
Never also meets a high-society family, and I believe they take her in, at least for a bit. To no one's surprise, they don't have the best intentions, and the matriarch of that family is, I believe, the main villain. I think Never believes that the matriarch may be her biological mother. I'm pretty sure Never later drugs the matriarch with some magical wine or perfume.
A magical mirror plays some sort of important role.
At the end of the book, Never leads the whole underground (or at least a lot of people) out above land, much to the discontent of the network, which tries to stop them. This gets rid of their magic, but I think gives them their facial expressions back. It turns out there are people living aboveground. For the epilogue, we are in the point of view of one of them, who notes that the people are a bit weird. They also note that Never, although not particularly attractive, is full of life and in a state of near-constant wonder.