It's easy to assume that Martin Luther King Jr.'s famous and endlessly influential "I Have a Dream" speech was pre-scripted, vetted by others, written and re-written. It was not. Instead it was given on the fly, when from the audience Mahalia Jackson urged King off his prepared text. The result has inspired millions. In this breath-of-fresh-air thinking about how to cultivate the kind of game-changing creativity everyone seeks, Stephen Nachmanovitch...