Fortune spends some time with Group M's CEO Irwin Gotlieb and concludes that this "bookish media buyer is king. ... We might just as well call him the $59 billion man." Last year, Gotlieb directed more than 16% of the world's $364 billion in global ad expenditures. He not only understands media better than most; he has the power to sway the industry to his vision. And lately that vision entails old media taking lessons from the web. In this profile of WPP Group's go-to media man, read how Group M is using online data to inform offline media buys.