This expression means to strictly follow a set of rules, laws, guidelines, or regulations. Some of these may be written in an actual book, but this term is used figuratively and also extends to following societal norms and conventional methods of doing things. When you do things 'by the book,' you follow one accepted method. It comes with less risk than 'thinking outside the box,' but can often be less rewarding in a professional context. In a social context, this expression is often used in relation to avoiding trouble with the law. Phrases that share a similar meaning include 'to the letter,' 'fair and square,' and 'follow protocol.'