Literature times 2: Author Robert Ludlum's best works?

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Well the 500 page+ novel "Sigma Protocol" by Robert Ludlum did keep me engaged and engrossed for many days. It's coming to an end. Have you checked the works by this author. I know that he has also been the author of Bourne series, many of which have had been converted to movies. But what are his other good works which are worth reading?