What was the main subject of the writings of alexander solzhenitsyn?

Prison camps and the violation of human rights in the Soviet Union.

A key work by Solzhenitsyn was the 3-volume set, The Gulag Archipelago (1973). Solzhenitsyn related his own experiences in a prison camp as well as what he researched from the experiences of others.   The Gulag Archipelago and most of his writings had to be published in the West, outside of the Soviet Union, where they were not allowed.  The only work of his that had seen publication in the USSR was One Day in the Life of Ivan Denisovich (1962),  which was a novel set in a 1950s Soviet labor camp.

a. prison camps and human rights in the Soviet Union