You can read it with a nice historical introduction at: http://www.bookofconcord.org/exhortation.php
This letter contains some of the many things Luther would have said at the Augsburg Diet had it been safe for him to move about the country. Adding to this the fact that it was written inside a huge, austere and creepy castle makes for some seasoned aesthetic appeal.
To be honest, I prefer this document to the Augsburg Confession and the subsequent Apology of Phillip Melanchthon. There is no doubt that Master Phillip is a brilliant scholar and a masterful theologian, but his writings lack the fire and ferment possessed by his co-reformer and friend Luther. At times Phillip can become quite monotone and repetitive, most especially in the longest section of the Apology known as "On Love and the Fulfilling of the Law."
Here is an exerpt from Luther's infinitely superior treatise:
"In a word, who can tell all the abominations that the indulgence, as a true and mighty idol, has caused in all the chapters, cloisters, churches, chapels, hermitages, altars, pictures, tables, nay, in almost all the houses and chambers, so long as there was money in them? One would have to read again the books that were written against them ten years ago or so. Now speak up, dear sirs! For this unspeakable thievery and robbery of money, and for this inconceivable number of deceived hearts and consciences, and for this terrible and abominable lie, this blasphemy of Christ's Passion, of the Gospel, of grace, nay, of God Himself, which have been committed by indulgences, all of you clergy are together to blame; not only you who have got money by them, but also you who were silent about it and looked on willingly at this raging of the devil. You talk of rebellion, of confiscation of monasteries, of the Turks! What are all these things together compared to you indulgence-vendors, when one thinks about it? It was a real Turkish army against the true Christian faith.
But which of you has ever once repented for this terrible abomination, or even sighed over it, or had a wet eye? And now, like hardened, unrepentant sinners, you will have it that you never did anything wrong; therefore you come together in Augsburg and want to persuade us that the Holy Ghost is with you and will accomplish great things through you, though all your life long you have done the Church nothing but harm, and that afterwards He will lead you straight to heaven with all these unrepented abominations - nay, with the abominations that you have defended - as though He must rejoice that you have served your god Belly so gloriously and laid waste God's Church so pitifully. This is why you have no good luck; and you will have none until you repent and amend your ways. "
It is impossible to read through the whole work and not SERIOUSLY lament the amount of corruption and absolute, maddeningly depraved wickedness that had infiltrated nearly every layer of the catholic ecclesia. God save us from these things.
If you have not yet read this document, I heartily recommend it.