Robert Jefferson Breckinridge's 24-page pamphlet entitled Two Speeches on the State of Our Country (1862) opens a window into the way that Northern Presbyterians viewed the Civil War, the South, and Southern Presbyterians. Breckinridge had been close to many Southerners throughout the early days of Old School Presbyterianism (1837ff.), but he could not tolerate the views they held on slavery and secession. Read these speeches and you will discover the reasons why.
Most pastors and seminary students are aware of Charles Hodge's three volume Systematic Theology. It is one of the most recognized works of the 19th century, and can be purchased here. But this set certainly was not the only systematic theology published by Presbyterians in the 19th century. Sometimes remembered are William Greenough Thayer Shedd's Dogmatic Theology (you can purchase the modern edition of this book here) and Robert Lewis Dabney's Systematic Theology (you can purchase the Banner of Truth reprint of this volume here). Very few people know that Robert Jefferson Breckinridge published a two volume systematic theology, entitled The Knowledge of God Objective Considered and The Knowledge of God Subjectively Considered.
Older systematic theologies weren't asking all the same questions we ask today, of course - but that's a big reason why they are so helpful to use. They ask questions we don't even know we need to be asking. So the next time you're wrestling with a theological question, dip into one of these volumes and see what riches you might find.