My top five are not ordinary classics but essential foundational works in the Reformed tradition.
a) Martin Luther’s Catechism: https://catechism.cph.org
It is known for its unique pastoral fervor in both life and practice. His distillation of the Ten Commandments is excellent.
b) Calvin’s Dedication to the Prince in his Genesis commentary: https://ccel.org/ccel/calvin/calcom01/calcom01.v.html…
Calvin’s prefaces are usually the most personal notes to dignitaries of the day. I wish we could restore such dedications, but they also summarize his general project.
c) Benjamin Warfield’s “The Plan of Salvation” is less than 100 pages, but it is one of the finest defenses of Postmillennialism in the 19th century: https://amazon.com/Plan-Salvation-B-Warfield-ebook/dp/B06XSNJB1F/ref=sr_1_4?crid=2FQ1IWIVYCWBG&keywords=the+plan+of+salvation+warfield&qid=1676508216&sprefix=The+Plan+of+Salvation%2Caps%2C84&sr=8-4…
d) Abraham Kuyper’s Lectures on Calvinism is the most thorough devastation of dualism. He continues in the spirit of Geneva. Shameless plug: I wrote the introduction to the newly printed version from Canon Press: https://canonpress.com/products/calvinism-the-stone-lectures/…
e) The Holiness of God by my old pastor, R.C. Sproul, remains one of the finest classics and expositions of Isaiah 6: https://amazon.com/Holiness-God-R-C-Sproul/dp/B002KCR0LS
