Complex sentence: Martin gradually extended his originally intended trilogy to seven volumes, the fifth of which, "A Dance with Dragons", took him five years to write before its publication in 2011; he has publicly hinted that the series might be stretched to an eighth volume.
Split into simpler sentences: Martin gradually extended his originally intended trilogy to seven volumes, the fifth of which, "A Dance with Dragons", took him five years to write before its publication in 2011. He has publicly hinted that the series might be stretched to an eighth volume.