What is the author's style in Catherine, Called Birdy by Karen Cushman?

Catherine's language is slightly more informal and modern than a girl of her time and education would have used, to make her story accessible to contemporary readers. For the most part, she uses everyday language that contemporary readers should be able to follow, as many words are the same. Certain expressions ("Corpus bones!" for example, which Catherine often uses as a curse), while not in modern usage, are easy to guess the meaning of in context.