Fantasy; Coming of Age
Technically Westmark is a fantasy novel because it takes place in a made-up land, though besides this, it's pretty much just full of realistic elements. Unlike most books in the fantasy genre, this one doesn't have a lick of magic in it—and this means that the conflicts our characters face are based in reality (though sometimes their circumstances are a little extreme… it's not every day that someone turns out to be a princess, after all).
As far as coming of age goes, this book isn't quite firmly rooted in the genre—this isn't just Theo's story after all—but insofar as Theo hangs out at center stage for a good portion of it, it's definitely a coming of age tale. After all, Theo goes from contented apprentice who happily follows orders, to rebellious printing press operator, ready to do anything for those he cares about. He certainly comes into his own as this book progresses, and really seems like an adult when it ends, and because of this, Westmark falls under the coming of age genre too.