Extracting topics is a good unsupervised data-mining technique to discover the underlying relationships between texts. There are many different approaches with the most popular probably being LDA but…
J. Weng, E. Lim, J. Jiang, and Q. He. Proceedings of the Third ACM International Conference on Web Search and Data Mining, page 261–270. New York, NY, USA, Association for Computing Machinery, (2010)