Two-and-a-half years ago, at a meeting in Cambridge, leaders of 42 of America's top libraries and research institutions decided that the time had come to build something together. But what was that thing? After a half hour, the group was able to agree on a single sentence: "It's a worthy effort, and we are willing to work together toward it." The "it" in question: a national, digital public library. If that moment was the Digital Public Library of America's conception, then today is its birth, with the launch of DP.LA, the effort's online home. (The U.Va. Li...