Thomas Watson originally intended A Body of Practical Divinity to consist of three works: "Body of Divinity", his commentary on the Westminster Shorter Catechism; "The Ten Commandments", an exposition on the summary of the mosaic law, and its applicability as a guide to the moral life of the Christian; and "The Lord’s Prayer", his exposition on the Lord’s Prayer as related by Matthew.

If there is any work among the myriad volumes of Puritan works that can properly be considered a Systematic Theology, Thomas Watson's A Body of Divinity
Written as a commentary on the Westminster Shorter Catechism, "A Body of Divinity" exposits the Cathechism, beginning with the question "What is the Chief End of Man?" Watson continues on to matters of Theology Proper (the Attributes of God), Anthropology (the Nature of Man), Christology, the Covenant of Works and the Covenant of Grace (Soteriology), and Eschatology (Last Things). For a study of Puritan thought and theology, "A Body of Divinity" remains a fantastic starting point.
Watson passionately defends the Ten Commandments as a foundation for Christian practice. “Obedience carries in it the life-blood of religion,” he says. “The rule of obedience is the written word. To seem to be zealous, if it be not according to the word, is not obedience, but will-worship.” Convicting without condemnation, Watson’s words inspire as he points the reader to God in Christ, and always with a view to dependence on the Spirit.
“The Ten Commandments are the rule of our life, the creed is the sum of our faith, and the Lord’s Prayer is the pattern of our prayer.” Watson concludes his magnificent work with a rich commentary on our Lord’s Prayer in the Gospel of Matthew. Avoiding the common prayer acronyms that saturate modern Christianity, he deftly builds a foundation for communion with God that requires our complete devotion.
Every attempt has been made to eliminate erroneous verse references present in earlier editions.

(From Goodreads)