Here is what I wrote on Wikipedia about this book (I am Jstanley01 on the Page History document there):
In what was likely Booth's most-recognized book, The Rhetoric of Fiction, he argued that all narrative is a form of rhetoric.
The book can be seen as his critique of those he viewed as mainstream critics. Critics according to Booth, who cites an abundant array of sources, that beginning roughly with Henry James began to emphasize the difference between "showing" and "telling" in fiction. And who have placed more and more of a dogmatic premium on "showing."
Despite the realistic effects that modern authors have achieved, according to Booth, trying to distinguish narratives in this way is simplistic and deeply flawed. He argued, citing authors of a variety of stripes, that they invariably both show and tell. And he observed that they appear to choose between the techniques based upon decisions about how to convey their various "commitments" along various "lines of interest."
Booth's criticism can be viewed as distinct from traditional biographical criticism (still practiced, especially among popular critics), the new criticism which argued that one can only talk about what the text says, and the modern criticism which argues for the "eradication" of authorial presence. Booth claimed that it is impossible to talk about a text without talking about an author, because the existence of the text implies the existence of an author.
Booth not only argued that, whether or not an author -- as distinct from the narrator of a text -- intrudes directly in a work, that readers will always infer the existence of an author behind any text they encounter. He also claimed that readers always draw conclusions about the beliefs and judgments (and also, conclusions about the skills and "success") of a text's implied author, along the text's various lines of interest:
"However impersonal he may try to be, his readers will inevitably construct a picture of the official scribe who writes in this manner -- and of course that official scribe will never be neutral toward all values. Our reaction to his various commitments, secret or overt, will help to determine our response to the work."
This implied author (a widely-used term that Booth coined in this book; who he also called an author's "second self") is the one who "chooses, consciously or unconsciously, what we read; we infer him as an ideal, literary, created version of the real man; he is the sum of his own choices"
Booth also spent several chapters -- which include numerous referrals to and citations from widely-recognized works of fiction -- describing the various effects that implied authors achieve along the various lines of interest that he identifies, and the pitfalls they fall into, depending upon whether or not the implied author provides commentary, and upon the degree to which a story's narrator is reliable or unreliable, personal or impersonal.
Booth detailed three "Types of Literary Interest" that are "available for technical manipulation in fiction":
"(1) Intellectual or cognitive: We have, or can be made to have, strong intellectual curiosity about "the facts," the true interpretation, the true reasons, the true origins, the true motives, or the truth about life itself. (2) Qualitative: We have, or can be made to have, a strong desire to see any pattern or form completed, or to experience a further development of qualities of any kind. We might call this kind "aesthetic," if to do so did not suggest that a literary form using this interest was necessarily of more artistic value than one based on other interests. (3) Practical: We have, or can be made to have, a strong desire for the success or failure of those we love or hate, admire or detest; or we can be made to hope for or fear a change in the quality of a character. We might call this kind "human," if to do so did not imply that 1 and 2 were somehow less than human."
In the 1983 edition of The Rhetoric of Fiction, in which a lengthy addendum to the original 1961 book was published, Booth outlined various identities taken on by both authors and readers: The Flesh-and Blood Author, the Implied Author, the Teller of This Tale, the Career Author, and the "Public Myth"; and, the Flesh-and-Blood Re-Creator of Many Stories, the Postulated Reader, the Credulous Listener, the Career Reader, and the Public Myth about the "Reading Public."
 The Rhetoric of Fiction 71.
 The term "second self" was brought into prominence by Kathleen Tillotson (1959), see Tom Kindt, Hans-Harald Müller, The Implied Author: Concept and Controversy (2006) p. 50).
 The Rhetoric of Fiction 74-75.
 The Rhetoric of Fiction 125.
 The Rhetoric of Fiction 428-421.