Peter Lombard. Four Books of Sentences AKA  Sententiarum Quatuor Libri (12-th century)

Thomas Aquinas. Summa Teologica (1265–1274) - Translated in English.

(Read on-line, Download as: PDF )

Sancti Thomae de Aquino. Summa Theologiae (1265–1274). Summa Teologica in the original Latin text

Hunter, Sylvester Joseph.Outlines of Dogmatic Theology (1895)

Volume 1, Volume 2, Volume 3 (Read on-line and download as DJVU and PDF pages )

Fiorenza, Francis Schuessler; Galvin, John P. (Editors). Systematic Theology. Roman Catholic Aspects (1991) (Read on-line).