This blog has written about the subject of sentence mining in learning a language twice, once on using word frequency lists to learn a language, and in the post about overcoming a language learning plateau. Another post also spoke about a source of free French literature that’s a good source of French sentences.
Lernhelfer.de is a collection of free online textbooks aimed at German secondary school students. However, they’re a good place to do sentence mining in German. The books are about a variety of subjects like biology, history, art, geography, and physics.
The German textbook is especially recommendable. There are a wide assortment of PDFs attached to it with litarary works throughout history. Some of these works are German translationsof works by non-German authors, including Mark Twain, Arthur Conan Doyle, Hans Christian Andersen, and Feodor Dostoevsky. Most of the authors are German, though, and some of them include the Brothers Grimm, Johann Wolfgang Goethe, Walther von der Vogelweide, Friedrich Nietzshe, and Franz Kafka.
Lernhelfer.de offers the chance to both increase one’s knowledge of German while also learning more about German history and literature. If you’re a German language enthusiast, this website (Scroll down for the books.) might be worth visiting.