I think it's the one that talks about 10,000 hours of work being required to be an expert in something....sorry that was outiers
https://www.businessinsider.com/malcolm-gladwell-explains-the-10000-hour-rule-2014-6
both are great, thought provoking books. Interestingly, the shortest medical residency is 3 years at about 60- 80 hours a week....