One of the most formative books I read early on in my Christian life was JC Ryle's classic, Holiness. I have read it several times since and continue to be blessed by it. If you haven't read it you should pick it up. If you have already read Ryle (or even if you haven't) here's another must read. Joel Beeke's books, Puritan Reformed Spirituality, is a great compliment to Ryle as it presses deep into Reformed spirituality. It is also a particularly helpful corrective to some of the ongoing reductionistic talk about godliness in the Christian life. In the chapter titled "Cultivating Holiness", Dr. Beeke gives one of the most clear and treatments of the subject that is easy to read and encouraging. Here's a taste where he presses on the reality of our holy standing before God in Christ as well as the holy conduct to be practiced in Christ.