One of my favorite books of systematic theology.
Eusden's 70-page introduction provides a brief biography of Ames, demonstrates his Augustinian and scholastic roots, describes his debt to the 16th-century logician Peter Ramus, gives a nice overview of Ames's teaching on each major doctrinal locus, and closes with a helpful visual flow-chart of the Marrow.
The Marrow itself is basically 240 pages of one- to five-sentence summaries of Ames's positions on most major doctrinal questions, with Scriptural proof-texts incorporated into the text. Here's an example, from Section XVIII of Book I, on "The Person of Christ, the Mediator":
27. There were in Christ two kinds of understanding: a divine understanding whereby he knew all things, John 21:17, and a human, whereby he did not yet know some things, Mark 13:32. So there were two wills, one divine, Luke 5:13, and the other human, with a natural appetite, Matt. 26:39. So Christ has a double presence, but the human presence cannot be everywhere or in many places at once.
The brevity of Ames's statements, and their occasional pith, are a large part of what make this book valuable. You don't have to wade through mountains of material to identify his conclusions; i.e., he doesn't qualify his claims a thousand times, which makes his provocations keener and also somehow more pastoral/homiletical. Add to this that the Marrow held greater sway in New England than any other theological textbook up until 1800, and that it presents a helpful snapshot of pre-Westminster Puritanism, and you have a book worth your time.