Buffett's investment approach can be gleaned from his writings and explanations of holdings in the Berkshire Hathaway annual reports. Mary Buffett, a former daughter-in-law of Buffett's, and David Clark, a family friend and portfolio manager, collaborated to write Buffettology: The Previously Unexplained Techniques That Have Made Warren Buffett the World's Most Famous Investor, a book that discusses his approach in an interesting and methodical fashion.