Next to John Owen's "Mortification of Sin" this is one of my favorite Puritan books on sanctification.
Marshall eloquently and biblically points to our union with Christ as the source of the grace of sanctification. With Christ as the fountain of this grace in the gospel, we have the means to walk dutifully toward godliness.
Along with Marshall's emphasis on union with Christ and the gospel principles and means of growth, I enjoyed his Direction 13 on the right use of the means of grace. Often, Reformed folk sound snobbish and unattractive when speaking about the means of grace, but Marshall has a tone that made me read this chapter carefully. It isn't that God doesn't work through other means, but the most ordinary and powerfully normative means of God's communicative grace are his Word, sacraments, prayer, praise, and fellowship.
This good is dense, but it a great read for those wanting a faithful, basic introduction to how the gospel drives one's growth in holiness.