Ryle's work is a classic Reformed defense of the pursuit of sanctification as an active enterprise against various late 19th century denigrations of that understanding. He is, in a way, a Puritan out of place in history (and easier to read than John Owen). In Holiness, Ryle stands against extremes among the Reformed and Lutheran that claim sanctification is all about faith, but also against the holiness movements of his day, arising in part out of a misdirection of Wesley's understanding of immediate sanctification.
He begins noting the difference between justification and sanctification: "In justification the word to be addressed to man is believe - only believe; in sanctification the word must be 'watch, pray, and fight.' What God has divided let us not mingle and confuse." At the same time each are integral to a complete understanding of the salvation won in Jesus Christ.
The book is a collection of addresses, not a completely systematic understanding, but his first three chapters (on sin, sanctification, and holiness) come close to that kind of completeness. His words are challenging and demand doctrinal understanding, deep personal awareness, and an application of doctrine to life. "Sanctification is that inward spiritual work which the Lord Jesus Christ works in a man by the Holy Ghost , when He calls him to be a true believer. He not only washes him from his sins in His own blood, but He also separates him from his natural love of sin and the world, puts a new principle in his heart, and makes him practically godly in life."
My criticism is two-fold (aside from the too constant Reformed denial of the possibility of holiness in this lifetime). Like the Puritans before him, Ryle makes holiness a mental experience, often ignoring the fact that we are embodied beings and might need direction in the physical realm. He also makes it too individual an experience, ignoring the communal call to holiness and the role of the church in achieving Christian maturity. As a spur to holiness this is a wonderful book, but it requires a "style of life" lived out in the physical world and amid Christ's body here on earth.