If you’ve ever watched “The Sopranos” or the 1997 movie “Donnie Brasco,” you might have wondered whether killing people is fun. After all, it seems fun to watch, at least on TV. Or maybe it’s not the killing that’s fun. Maybe it’s the planning, the camaraderie, the petty squabbles, and the disdain for normal bourgeois existence that holds our attention. Demanding protection money and dealing drugs seems fun, too. If you’ve had such thoughts, have I got the social scientist for you. Beginning in 1989, Sudhir Venkatesh, now a professor at Columbia University, followed around a Chicago gang leader, identified only as J.T., for about six years, ostensibly to complete his sociology Ph.D. at the University of Chicago. Though Mr. Venkatesh did draw on his experiences in his dissertation, and in several much-discussed academic papers, the final result of his sojourn is “Gang Leader for a Day: A Rogue Sociologist Takes to the Streets” (Penguin Press, 320 pages, $25.95). If you’ve read the best-selling “Freakonomics,” Mr. Venkatesh should be a familiar figure: He was the guy who hung around young drug dealers, discovered that most of them don’t earn very much money and still live with their mothers, and asked why anyone would get into the business in the first place.