When Barack Obama came to prominence as a presidential candidate, his Chicago background—in particular, his efforts as a "community organizer"—reignited an interest in Saul Alinsky (1909-72), the hard-charging activist whose 1971 book, "Rules for Radicals," was said to have had a formative influence on Mr. Obama's thinking. Some critics worry that Alinsky's ideas guide Mr. Obama even today, in the White House. About such matters Nicholas von Hoffman cares little. But about Alinsky himself Mr. von Hoffman cares a great deal. He knew Alinsky, worked with him for 10 years in Chicago community groups and now offers a portrait of him in "Radical."