In 1530 Luther wrote a preface to a book written in the previous century by a Christian man who had been in slavery to the Muslims for 20 years. At this time in history, the Muslims were a dire threat to the West, yet very little accurate information on Islam was readily available. Luther had a deep interest in understanding Islam, and had read as much as he could get his hands on. He complained that many of the books refuting Islam "eagerly pick out the most shameful and absurd passages from the Koran, which provoke hostility and are able to move the multitude to hatred" (LW 59:258) while ignoring or concealing "the good passages" in the Koran. Why? Because these are harder to refute. The author of the book Luther wrote the preface for gained Luther's respect because,
"...he narrates [the details of Islam] in such a way that he not only recites their evils but also sets their best parts alongside, and he preaches those things in such a way as to rebuke and scold our people by comparison with them. Yet he does not approve of them as if they were pious deeds, but he refutes them with courage and vigor, inasmuch as was possible at that time. Indeed, these are sure signs of a candid and sincere heart, which writes nothing from hatred but tells everything from love of the truth. You see, whoever does nothing but find fault with his enemy, denouncing his shameful and absurd aspects, does more harm to his case than good. What is easier than to denounce the manifestly shameful and dishonorable things (which, in any case, refute themselves)? But to refute the good and honorable things, after taking away their splendor, is what benefits one's case; this is what removes stumbling blocks and strips the false form off the angel of light [cf. 2 Cor. 11:14], making him odious by virtue of his own turpitude and his robbery of the light." [LW 59:258-259]The book by the way, was George of Hungary, Book on the Ceremonies and Customs of the Turks. I searched around a bit a couldn't find a free link, so if anyone can come up with a link, I'd appreciate it. For $35 you can read it here.