Generally speaking, rhetoric is the study of the way language is used to make an argument or to persuade readers or listeners. When people talk about rhetorical language, for example, they are referring to words and turns of phrase that pose facts and ideas in a certain way so as to give a reader or listener a certain impression. Instructors who teach rhetoric normally want students to become aware of how language is used in the texts they read and how they can improve their own work with a stronger understanding of rhetoric. To choose the best books on rhetoric, you first should think about why you need this book, such as for a class, as a reference guide, or for general information. It also is a good idea to choose books on rhetoric that are written in a style you like, by authors who are reputable and established in related fields, and which also are current or timely.
Thinking about why you need books on rhetoric is the first step to making the best choices. If you are taking a class on rhetoric, then you should wait until you receive a syllabus from your instructor. Chances are that he or she already has a number of texts picked out for your class. If you are interested in doing some supplementary readying that can help you better to understand class material, however, you should contact your instructor for recommendations of books on rhetoric that include relevant material and which are written for readers at your skill level.
Outside of school, choosing the best books on rhetoric is a matter of using critical thinking. If you are interested in having a reference book, make sure that you find a text that is published by a respectable academic press. In most cases, this kind of rhetoric book might have words such as "encyclopedia" or "handbook" in its title. It also is important to keep in mind that reference books on rhetoric commonly are aimed at certain fields or professions. Some books, for example, might be written for students of Shakespeare, while other books might be written for journalists or legal professionals.
You also should consider an author's style and credentials when it comes to choosing the best books on rhetoric. For example, if you are looking for a light, informational read, you might want a book written by a literature professor who understands rhetoric and who has a sharp sense of humor. If you are interested in learning from a foundational text, however, you might choose The Art of Rhetoric by Aristotle, which many experts consider one the most important books on rhetoric ever written.