Every book in the New Testament section of the Bible has a human author (i.e., a man who either wrote the revelation down himself, or spoke it to a scribe who did) assigned to it except one, 1 the letter to the Hebrews.  Because of where it's placed following Paul's epistles, and some of the language in it, it is a widely held belief by Bible expositors that its author is, indeed, the apostle Paul.  The following essay, by Pastor Jerry Lockhart, lays out a strong doctrinal case for why this can't be.

Click to read the essay.