Complex sentence: The market for cheap magazines diminished when buyers went to cheap books instead--one factor in this was that the content included in both formats crossed over--authors also found themselves abandoning magazines, and writing for the paperback market.
Split into simpler sentences: The market for cheap magazines diminished when buyers went to cheap books instead. One factor in this was that the content included in both formats crossed over--authors also found themselves abandoning magazines and writing for the paperback market.