Didn't Saint Paul write all the letters attributed to him?

For everybody who didn't get the memo: Biblical authorship is tricky. It can't compare to contemporary authorship, defined against forgery and plagiarism—and in favor of copyrights and royalties. Scripture writers didn't claim rights over their work. They didn't seek fame or a livelihood for their efforts. Ancient writers sought to establish the mantle of authority, rather than authorship, for what they set down. So they often wrote under the auspices of existing schools of thought. To the ancients that was not skullduggery; that was how it was done. The contemporary filmmaker who pays "homage" to earlier directors that contributed to his or her vision is invoking a similar liberty.

In the Old Testament, scholars presume four schools of writers contributed to the five books of the Bible commonly known as the "Law of Moses." The authority of Moses is invoked, but even the rabbis don't hold that Moses penned Genesis through Deuteronomy. In the same way, three generations are believed to have contributed to the Book of Isaiah—one being the 8th-century B.C. prophet himself, whose scroll was extended by later students and admirers. Many Hebrew texts were added to or edited by later compilers in this way. But it's in the New Testament that authorship gets really interesting.

Saint Paul was the first contributor to what would be known one day as the New Testament. He wrote a generation before there were gospels, so his early witness to Christian beliefs and practices is quite valuable. Most Catholic and many Protestant scholars hold at least seven of the 14 letters attributed to Paul to be authentically his: 1 Thessalonians, Galatians, 1 and 2 Corinthians, Philippians, Romans, and Philemon. If you read them in that order, you get the style, personality, theology, and viewpoint of a single unique letter-writer.

Three other letters are routinely classified as Deutero-Pauline. This means they reflect Paul's ideas but also reveal another hand, perhaps a student of his. The Deutero-Pauline letters are: 2 Thessalonians, Colossians, and Ephesians. Three more letters are hotly debated but widely regarded as non-Pauline. These are the so-called Pastoral Letters: Titus and 1 and 2 Timothy. A 14th letter once credited to Paul, Hebrews, is now universally regarded as by another author.

Truth is truth, no matter who says it. Because the authorship of Paul's letters was debated even by the church fathers, who nonetheless put them in the Bible, we might honor their assessment that the words are God-inspired, even if their author sometimes remains a mystery.

Scripture
The Pauline letters are best read chronologically: 1 Thessalonians, Galatians, 1 and 2 Corinthians, Philippians, Romans, Philemon, 2 Thessalonians, Colossians, Ephesians, Titus, and 1 and 2 Timothy. Read the Acts of the Apostles for background.

Online resource
A historical introduction to the Pauline Epistles

Books
How to Read the Bible by Richard Holloway (W.W. Norton & Company, 2007)
Paul the Letter-Writer by Jerome Murphy-O'Connor, O.P. (Liturgical Press, 1995)