The Summa Theologica of Saint Thomas Aquinas (5 vol.): I consider Thomas Aquinas both the greatest philosopher and theologian of all time, even though I harbor many disagreements with the man.  This might be the most important piece of philosophical literature ever written.

Reformed Dogmatics- Herman Bavinck: In my opinion Bavinck is the greatest of all Reformed dogmaticians apart from Calvin.  This is one of those groundbreaking works that belongs in every theological library.

The Mystical Presence- John Williamson Nevin:  Nevin is a Reformed Catholic par excellence.  If you want to better understand the centrality and nature of the Lord’s Supper in corporate worship, this is the book to read.  This is the definitive work on Calvin’s eucharistic theology, and a very great place to start in understanding Nevin and Mercersburg.

Calvin’s Institutes of the Christian Religion: What else can be said?  Here is Reformed theology at its best.

Commentary on the Heidelberg Catechism- Zacharias Ursinus