All letters officially included in the New Testament that weren't written by Paul.
The other Epistles are letters that are said to have been written by prominent early Christian figures- such as John the Apostle, Peter the Apostle, and James the Brother of Jesus- though scholars believe none of them are written by their traditional authors. They were likely composed between the late 1st century and the early 2nd century CE. Their inclusion in the New Testament canon reflects the efforts of early Christian communities to preserve and circulate writings that were deemed authoritative and spiritually edifying.
Regardless of religious beliefs, a secular examination of the non-Pauline Epistles contributes to our understanding of the diverse and evolving nature of early Christianity. These letters offer valuable insights into the complex interplay of theological ideas, ethical teachings, and communal practices within early Christian communities.
Two standout books are Hebrews and Revelation, included here because they are technically letters, but are standalone books.