What is the author's style" of a book?"
When thinking about style, first consider the clothes people wear. Do you know someone who always dresses in black or who never wears plaid? Maybe the clothes are pure vintage (from the local thrift shop, perhaps) or very modern and straight from the most fashion-forward stores. Whatever they wear, people choose clothes to express themselves and their individual "style."
Authors have style, too, but it's called literary style or the writer's voice. Writers express their ideas in specific ways by using just the right word and putting it in just the right place. A literary style reflects the author's personality and the purpose for the writing (that is, the idea to be expressed). For example, you might have two writing assignments — a short story for 10-year-olds and a research paper for your doctorate degree. The purpose of each assignment requires a different tone, word selection, and sentence structure. Which one would be casual and conversational? Formal and academic?
Other elements that contribute to writing style include the rhythm of the language, figures of speech, punctuation, and character development.