Several years ago, I found myself sitting at a desk in the main reading room of Stanford University’s Hoover Library. The box in front of me contained thousands of unopened letters, their thin red, white, and blue envelopes stacked indiscriminately. All were postmarked in the winter of 1972, and written in languages of the former Soviet Union: Russian, Latvian, Lithuanian, Estonian, Ukrainian, Kazakh. Most notably, all were addressed to Angela...