The Oxford Dictionary defines belles lettres as “essays, particularly of literary and artistic criticism, written and read primarily for their aesthetic effect.” In a more general sense, literature considered as a fine art. In French it literally means “beautiful letters.” More old-fashioned bookstores and library employing the Dewey Decimal system usually have a section devoted to belles lettres, which to the young, inquisitive mind that I was, remained an ambiguity.
A New Handbook of Literary Terms by David Mikics notes on belletristic style: “A piece of prose writing that is belletristic in style is characterized by a casual, yet polished and pointed, essayistic elegance. The belletristic is sometimes contrasted with the scholarly or academic: it is supposed to be free of the laborious, inert, jargon-ridden habits indulged by professors.”
In other words, the belles lettres style is individual in a sense. This is one of its most distinctive properties. Individuality in selecting language means (including stylistic devices), extremely apparent in poetic style, becomes gradually less in, let us say, publicist style, is hardly noticeable in the style of scientific prose and is entirely lacking in newspapers and in official styles.
My understanding is that prose fiction differs from general fiction in the style of prose, which has the quality of poetry and drama. Owing to this rigid criterion, it is no wonder the works of Gabriel Garcia Marquez, Toni Morrison, Mikhail Bulgakov, Virginia Woolf, and F. Scott Fitzgerald are categorized as belles lettres. The common ground of their works is that a peculiar individual selection of vocabulary and syntax, or a kind of lexical and syntactical idiosyncrasy is employed to create a context unique within the work itself.