Complex sentence: Early in his career, especially with his books The Triumph of Conservatism and "Railroads and Regulation", Kolko used a revisionist approach as a way of analyzing history and was considered a leading historian of the New Left,;.
Split into simpler sentences: Early in his career, beginning with his books "The Triumph of Conservatism" and "Railroads and Regulation", Kolko used a revisionist approach as a way of analyzing history. Soon he was considered a leading historian of the New Left,;.