Summa Theologica, Part I (Prima Pars)
Theologica is the premier work of Catholic theology, a systematic and organic treatment of several thousand important theological questions, ranging from God, the Trinity, and the nature of Christ, to the nature and psychology of the human person and the nature and mission of the Church. St. Thomas considers creation in its natural light, as well as under the operations of grace, including discussions of morality, redemption, the sacraments, and the operation of divine providence. The scope of topics is enormous, and these few hardly do justice to the contents.
Love this book.
If marooned on an island, I'd want this book and the Bible. Soon after , I'd build a raft to tell the world of the love of God.