Wes White, pastor of New Covenant Presbyterian Church in America in South Dakota, compiled a list of older Reformed books that are available online for free. Most of these works are from the Reformation and post-Reformation period but continue up to a few resources from the beginning of the 20th century. Many of these works are from French Hugeunot pastors or Scottish and English puritans. You can find some classic works on this list such as Thomas Boston's The Fourfold State of Man, Herman Witsius on the economy of the covenants, and most of John Owen's complete works. I highly recommend bookmarking this page and saving some favorites on Google books or saving/printing some of these works to read over. Some of the titles are still in the original languages and so require some knowledge of French, Latin, Dutch, or German but most of them are in English.