The Bible is the story of the House of Judah. It is important because that the family that Jesus was born into. His mother Mary and her husband Joseph were both of the House of David and carried royal blood. It was important that the Bible be published for all of the world, but they aren't the only tribe. In prophecies the House of Joseph (Ephraim and Manasseh) are listed along with Judah. Ephraim and Manasseh were the major tribes and had the most members. Great leaders like Joshua belonged to the tribe of Ephraim. The Northern Kingdom was called Ephraim as the Southern Kingdom was called Judah. Ephraim is supposed to fight against and then unite with Judah. So where are their records? We have some remnants of their records in the Book of Chronicles.
Before they were taken away to what is now Central Asia (Afghanistan, UZBEKISTAN, and Turkmenistan) by the Assyrians, they were given a chance by King Hezekiah of the Southern Kingdom to come to Jerusalem to worship in the temple there. Most of them ignored his invitation and continued to worship false gods. However, in 2 Chronicles 30:11 it says "Nevertheless divers of Asher and Manasseh and of Zebulun humbled themselves, and came to Jerusalem."
In 1 Chronicles 9:3 it says "And in Jerusalem dwelt of the children of Judah, and of the children of Benjamin, and of the children of Ephraim, and Manasseh."
So the Bible establishes that Ephraim and Manasseh had members in the city of Jerusalem. Lehi either came from the first or second waved of settlers into Jerusalem. Since he did not know he was from Manasseh, he probably was from the first group who came, possibly to escape King Ahab's evil or to escape one of the horrible famines. These settlers remained great leaders even in the Kingdom of Judah.
One of the major spiritual leaders who lived when Lehi was a child was Huldah who was in charge of a school for women in Jerusalem. She was of the House of Ephraim and a relative of Jeremiah, whose mother was from the tribe of Ephraim.