A Day in the Life of Ivan Denisovitch is excellent, and I agree with David that it is a good place to start.
Two superb short novels by Solzhenitsyn are An Incident at Krechetovka Station and Matryona's House. They are available in a single volume called We Never Make Mistakes (New York: W.W. Norton, 1996).
Warning: Matryona's House will make you cry.