The contents of a book revolve around the subject, and develop one or more central ideas. For nonfiction, a reviewer analyzes how well the contents of a book address the central idea, the strength or weakness of supporting ideas, and the relevancy of collateral ideas or implications.
In fiction, themes develop through character, setting, and plot; a reviewer evaluates the relative success or lack thereof of these fictional elements. Think about these questions: What is the setting, or place and time, of the story? Does the setting reflect or contrast with characters and plot? Are characters fully or minimally developed? Does character development increase or deteriorate as the action proceeds? Is the plot sequenced chronologically, or otherwise? Does tension build or deflate as the story progresses?
Note how David Milofsky discusses the effectiveness of the contents of Reynolds Price's Noble Norfleet: "Although there are spots of lyricism-and for the first third of the book, Price's narrative has the drive and tension of some of his better work-overall, Noble Norfleet sags beneath its unlikely premise and even more unlikely hero . . . It seems likely that Price was trying to say something here about the relationship between sexuality and madness, about the necessity not only of nursing others but of caring for oneself, of showing Noble as some kind of paradigm, hence his name. But, sadly, the novel succeeds in none of these aims."
Remember that details about the plot and characters in a book are revealed by the reviewer only to support the purpose of the review. Certainly, a review should not give away a book's ending, nor should it be a simple summary of events and characters. The reviewer's job is not only to report highlights but also to respond to the ideas and techniques evident in the book.