Police have to follow the law and departmental guidelines. In some cases, they have specific codes that they use to refer to the situation, and they refer to it by that code instead of saying what it is. The penal code refers to laws, and they might reference sections of the penal code when saying that someone violated such and such a law. To find out what specific law they are talking about, or what specific situation, you would need to know the jurisdiction or city and look in the penal code or departmental guidelines for that locality. The book also states the penalty, which might be where the term "throw the book at" someone comes from.
If you're talking about the police in real life (as opposed to in some book) the answer to this is that they are referring to the legal codes of their jurisdiction. States typically have their laws divided up by their subject. So you would have some section of the law be about burglary, for instance. Then there would be subsections of that law that fill in the details about that crime. These sections and subsections are what police (or, more likely, prosecutors) are referring to.