Just as a matter of interest, does Gladwell cite the Cambridge Handbook of Expertise and Expert Performance? It's also available from Amazon, but a touch more pricey. The handbook was the result of a long term, in depth study by a group of Universities, including Cambridge. There was coverage in the press when it was published (eg see New Scientist), it's very relevant to Gladwell's topic and its very well researched.
Here's one of the reviews quoted on this site: "The Cambridge Handbook of Expertise and Expert Performance makes a rather startling assertion: the trait we commonly call talent is highly overrated. Or, put another way, expert performers "whether in memory or surgery, ballet or computer programming" are nearly always made, not born. And yes, practice does make perfect." --Steven D. Leavitt and Stephen J. Dubner