This is a great place to look for monographs from antiquity (e.g., Athanasius, Basil the Great, Augustine), the Middle Ages (e.g., Thomas Aquinas), Reformation (e.g., Latimer, Luther), Romantic period (e.g., Schleiermacher), and Modernity (e.g. Kierkegaard). There are even "secular" works (e.g, Tolstoy). Many downloads are in (downloadable) PDF format and others in (read online) plain text.
Here's an updated view of the categories: