Why Generalists Triumph In A Specialized World
David Epstein wrote this book to address a few things, the first being the notion that you have to start early. One of the early contrasts noted in the book is between Tiger Woods and Roger Federer. Tiger was swinging a golf club as early as 10-months old and has (essentially) only played golf since then. Contrast this with Roger who had tried skiing, wrestling, swimming, skateboarding, basketball, handball, tennis, table tennis, badminton, and soccer. If it had a ball, Roger probably tried it.
Range goes on from there to show example after example of the power that broad knowledge and skill-set can have in a large number of fields. It doesn't discount the value of specialists; a number of fields rely on the ability to do one thing and to do it extremely well (e.g. surgeons). Overall, however, it attempts to debunk the modern-day notion that you need to specialize in one thing and one-thing only. This is especially prevalent at the collegiate level where barely-adults are asked to commit their lives to a direction (at the exclusion of the others).
The second thing Range addresses is the notion that it's too late to start or to change. There are a number of examples provided in the book of people who started "late" and went on to be successful in their fields. These include Steve Nash (started playing basketball at thirteen before going on to win the NBA MVP award, twice!) and Sviatoslav Richter (began formal piano lessons at twenty-two and became one of the greatest pianists of the twentieth century).
Compare yourself to yourself yesterday, not to younger people who aren't you.