The word meaning letter that is in titles of books of the Bible's New Testament is "Epistle". The New Testament contains several books that are referred to as "Epistles", which are essentially letters written by early Christian leaders to other communities or individuals. The Epistles in the New Testament include letters written by Paul, Peter, James, John, and Jude, and they cover various topics such as theology, ethics, morality, and admonishments. Some of the most famous Epistles in the New Testament include Romans, Corinthians, Ephesians, Philippians, and Colossians.