Lincoln once noted how the printing press spread knowledge by making works widely available that had previously been the province of a privileged few. The same is true when primary sources are collected, transcribed, and published; when exhaustive reference works are produced; when scholars leave published books and carefully organized research files; and when interest in a subject grows to the point that entire institutions--libraries, journals, and museums--are devoted to assisting its students. The main problem with studying Lincoln is not finding sources, but choosing which sources to follow. A