Which letters represent Moses' significance in history?
The first five books of the Old Testament, the Pentateuch, are commonly referred to as the Books of Moses. He first appears in Exodus where he was born in Egypt and raised in the house of the ruling Pharaoh. In the same book, he leads the Hebrews through the Plagues which afflicted the Egyptians and out of Egypt toward Canaan. In Numbers, the people are numbered and a portion of the Mosaic Law is set forth. That law is described in more detail in Leviticus. Finally, Deuteronomy is Moses' farewell address to the Hebrews, as he is not allowed to cross over with them into the promised land. He is allowed to see it from Mt. Nebo, but not to enter it. There are no "letters" attributable to Moses; all the letters are in the New Testament and were written by followers of Jesus.
Because of his leadership of the Hebrews, Moses is considered the second most important Hebrew historical figure, after Abraham.