The Religious Affections by Jonathan Edwards
I finally got back into the reading swing a few months ago and first on my list was a book that had been on my list for a long time, The Religious Affections by Jonathan Edwards. This book, even after having finished a complete reading, is so monumental that it would require several more readings, at a much slower pace, to even begin to comprehend it's value. First published in 1746, written around the time of the Great Awakening when "affections" were running wild (many people would have a dramatic "religious awakenings" with loud wailing and moaning but not a true change of heart), this book must have been seen by the people of North Hampton at the time as quite a controversial book. Today, The Religious Affections has the honor to be listed among the classics delivered by some of the greatest theologians, but if read in context of today's culture, and viewed as being directly applicable today, it might be seen as even more controversial today than it did in the late 18th century. Still, it's truths are so relevant, it's pious statements so profound, it tends to show how far we have come (or how far we have slid) from the "religion" of the Great Awakening. Where Edwards was once trying to discern true affections from Pharisaical outcries, we the church in the 21st century are similar to the 18th century church of North Hampton in some respects. We have and show almost no true affection in worship to God, a breaking of the will by the heart, for a God who deserves the utmost adoration for every breath we take, and yet we posses more entertainment emotion (for lack of a better phrase) than any generation in previous history.
As the book opens, Edwards puts forth nine evidences that true religion lies much in the heart of the affections. In seminary (of all places) it has often been said to me that a mature Christian needs both the head and the heart, both knowledge and true affections towards God. If you are in the camp that uses "knowledge puffs up but love edifies" (1 Corinthians 8:1) to excuse yourself from study you are missing half of what Paul is saying, and the same is true to those who only seek after knowledge. Any surface reading of scripture clearly shows that God insists on both, and Edwards certainly agrees. "He that has doctrinal knowledge and speculation only, without affection, never is engaged in the business of religion." 
In these nine evidences Edwards lays out his thesis and speaks directly to the church of the 21st century.
That religion which God requires, and will accept, does not consist in weak, dull, and lifeless wishes, raising us but a little above a state of indifference: God, in His word, greatly insists upon it, that we be in good earnest, "fervent in spirit," and our hearts vigorously engaged in religion (Romans 12:11) and to "Be ye fervent in spirit, serving the Lord... serving the Lord thy God with all thy heart, and will all thy soul?" (Deuteronomy 10:12).
While we certainly can claim we don't have dull and lifeless worship services (in fact we can claim the opposite since our worship "production" can rival that of the Discovery Channel at this point), we can still have a lifeless and dull heart. Paul in Romans 12 isn't saying the dB rating of the worship should be vigorous, he is saying that "our hearts [should be] vigorously engaged" in worship. John takes it one step farther when talking about the church in Laodicea saying that Christ utterly detests a lukewarm church (Revelation 3:16).
I would highly recommend The Religious Affections by Jonathan Edwards to anyone who might be interested. It certainly was a challenging read, it wasn't the most straight forward easy to read pop-Christian publication that tends to make the rounds today, but I wouldn't expect it to be either. Books that we fully understand from a quick initial read probably don't further our understanding in the subject at hand and Affections is one of those pieces of literature that could be read over and over again.
 Jonathan Edwards, The Religious Affections, (Carlisle, CA: The Banner of Truth Trust, Versa Press, Inc., 1986), 30, 27.