What was the first book to be translated in maori?

The first book to be translated into Māori was "The Gospel of St. Matthew" in 1815. It was translated by Samuel Marsden, a missionary from the Church Missionary Society.

It's worth noting that while the Gospel of St. Matthew was the first book published in Māori, there were other texts that were translated earlier, though not published. For instance, the Lord's Prayer was translated in 1814 by Thomas Kendall, another missionary.