Felicita is rich, but sheltered (think Jasmine from Alladin) and wants to escape. When her brother wants to marry her to a distant family, she decides it's better to be poor. She runs away, finds some help with some street kids, and gets involved in a plot against her own wealthy family.
That's a quick little summary, but the unique fantasy elements are what makes this book interesting. Sure, there are vampires, but they're called bats and you're not quite sure what's going on when they are first mentioned. There's violence in Pelimburg, as well as strange, dark relationships between the characters.