One of his other (not as well know--but equally valuable) works which was especially useful to me is Sacred Dissertations on The Apostles' Creed (2 volumes).
Hard to beat as an overview, a "big-picture" of Reformed Covenant Theology; then again, I may be slightly biased, since that was kind of the book that "made it all click" for me, as it were.
Witsius was the consolidator of the covenant theology of the Reformation, which was intended to bring together the Voetians and Cocceians in the Netherlands.