Complex sentence: By the mid- 1930s he was a prolific and successful writer of fiction for slick magazines like the "Saturday Evening Post"; during this period Marquand began producing a series of novels on the dilemmas of class, most centered on New England.
Split into simpler sentences: By the mid- 1930s he was a prolific and successful writer of fiction for slick magazines like the "Saturday Evening Post". In the late-1930s, Marquand began producing a series of novels on the dilemmas of class, most centered on New England.