April 28, 2006, 6:24 a.m. Not for the Faint of Heart Robert Spencer asks the hard questions about Islam...and answers them. The Politically Incorrect Guide to Islam (and the Crusades) , by Robert Spencer (Regnery, 233 pp., $19.95) I t is often said that in order to keep polite company polite, we must refrain from speaking of religion and politics. Yet, the two are not equals in the hierarchy of politesse. Political debate may be unwelcome in many settings, but no one clears the room by observing that the great totalitarian evils of the 20th century, Communism and fascism, were directly responsible for incalculable carnage. Not so when it comes to religion — or, at least, one particular religion. The past three decades have borne witness to a rising, global tide of terrorist atrocities, wrought by Muslims who proclaim without apology — indeed, with animating pride — that their actions are compelled by Islam. Nonetheless, the quickest ticket to oblivion on PC's pariah ex