Robert Liddell, the English novelist whose close friendships with Elizabeth Taylor and Barbara Pym influenced the course of the English novel, was perhaps best-known for his wonderful Treatise on the Novel (1947). In the late ’sixties, Wayne Booth arranged for its reprinting in a single volume along with Some Principles of Fiction (1953) by the University of Chicago Press. Although long out of print, it is worth hunting down. It is perhaps the best one-volume introduction to the novel, written in short bursts of aphoristic and shrewd opinion.
Liddell says that there are only two categories of novels: (1.) “Novels which call for serious literary criticism.” (2.) “Novels which are beneath serious criticism.”
The first category contains two subdivisions: (a.) “Good novels.” (b.) “Novels which might have been good.” To the second division Liddell consigns books that are “bad, uneven, or technical failures,” although they are written by writers with “minds of the necessary sensibility.” This division would also include the botched attempts by writers who earlier or later engineered good novels. The good novelist is anyone who has somehow managed to write a single good novel, but his other novels also merit study on the basis of his exceptional achievement. And then there are the novelists of eternal promise, who never quite put it all together in a single book—Aldous Huxley, Truman Capote, John Updike, Salman Rushdie, Paul Auster.
Liddell subdivides the novels beneath serious criticism into middlebrow and lowbrow books, but unfortunately he says little more to clarify either division. Distinguishing between them, he comments, is “like establishing the precedence between a flea and a louse.”
In his 1964 interview with Playboy, Nabokov was characteristically sharp-toothed:
 Robert Liddell on the Novel, ed. Wayne C. Booth (Chicago: University of Chicago Press, 1969), p. 10.
 Vladimir Nabokov, Strong Opinions (New York: McGraw-Hill, 1973), p. 41.