Paul wrote the majority of New Testament letters. These almost exclusively take the form of one or more persuasive theological arguments buttressed by proofs of various sorts, usually sandwiched between greetings and salutations. Peter’s first epistle follows a similar pattern, as does Hebrews. Others, like Jude and 2 and 3 John, were written with a single evident concern or focus.
James, not so much. He is all over the map.