Complex sentence: For many years he was head of the media company Australian Consolidated Press and previously of the publishing and bookselling firm Angus & Robertson, and in those roles he was "one of the most dominant figures in Australian publishing from the early seventies".
Split into simpler sentences: For many years he ran the publishing and bookselling firm Angus & Robertson and later he headed the media company Australian Consolidated Press. In those roles was "one of the most dominant figures in Australian publishing from the early seventies".