The Holiness of God by R.C. Sproul is a modern classic.  In fact, I've thought about purchasing a copy for every person in our church (you'll see why in a moment). If you commute or regularly have time to listen to audio books you'll want to take advantage of this deal.  ChristianAudio.com is  offering the audio book available for free during the month of March.  Christianaudio.com is a great company that puts out quality work.

It also appears that Ligonier Ministries (of which Sproul is the president) will send you a free copy (you pay shipping) if you are not currently on their mailing list.

Finally, if you're ever interested in investing some money in giving a gift that will build up believers and be a good tool in reaching unbelievers the deal going at Books by the Box is quite good.  You can purchase 12 copies for $28 or 48 copies for $80.

Finally, here are a couple of quotes from the book to whet your appetite.

The issue is not why does God punish sin, but why does He permit the ongoing human rebellion? What prince, what king, what ruler would display so much patience with a continually rebellious populace? ...We forget rather quickly that God’s patience is designed to lead us to repentance, to give us time to be redeemed. Instead of taking advantage of this patience by coming humbly to Him for forgiveness, we use this grace as an opportunity to become more bold in our sin… The supreme folly is that we think we will get away with our revolt.” (p. 117)

and

If ever there was a man of integrity, it was Isaiah ben Amoz. He was a whole man, a together type of a fellow.  He was considered by his contemporaries as the most righteous man in the nation. He was respected as a paragon of virtue. Then he caught one sudden glimpse of a holy God. In that single moment, all of his self-esteem was shattered. In a brief second he was exposed, made naked beneath the gaze of the absolute standard of holiness. As long as Isaiah could compare himself to other mortals, he was able to sustain a lofty opinion of his own character. The instant he measured himself by the ultimate standard, he was destroyed—morally and spiritually annihilated. He was undone. He came apart. His sense of integrity collapsed.

(HT: Justin Taylor)