Most authors, regardless of the particular language in which they write, do not use a "non-standard" form of that language throughout a novel except for dialogue, quotations, and isolated phrases or sentences. African American English (AAE), is essentially a set of dialects of English. If a novel is set in the African American community (as most of James Baldwin's novels are), a writer generally may employ AAE not for a third-person narrative but for at least some of the dialogue among characters who speak AAE. However, I tend to think that this, a realistic portrayal of the way people speak, would not be defined by most specialists as a "literary device." To give a single example, a literary device would be something such as the use of irony—in other words, a literary technique that exists independently of the specific language or dialect an author is employing.
Perhaps any issue regarding Baldwin's (or other authors') employment of AAE can be understood better if we analogize it with the use of dialects in languages other than English. For instance, in Mario Puzo's The Godfather, many of the characters speak the Sicilian dialect, which is quite different from standard Italian. Though the novel is written in English, Puzo uses Sicilian words in various quotations. But even if he had written his novel in Italian, Puzo would almost certainly have used standard Italian for his third-person narrative, and probably for most of the dialogue as well. Similarly, African American writers generally use standard English, though novels written in the first person sometimes employ AAE, such as Alice Walker's The Color Purple.