According to the author of 1 Kings, Jeroboam was a king who fought battles and ruled over his people, and all the things he did are written in another book that tells stories about the kings of Israel. Christians understand this to mean that many important stories about kings and what they did are kept in special books, just like how the Bible keeps important stories about God and people.
1 Kings 14:19: And the rest of the acts of Jeroboam, how he warred, and how he reigned, behold, they [are] written in the book of the chronicles of the kings of Israel.
It contradicts the implication that Nathan wrote about the acts of the kings, while 1 Kings 14:19 suggests writings by prophets.
1 Chronicles 29:29: Now the acts of David the king, first and last, behold, they [are] written in the book of Samuel the seer, and in the book of Nathan the prophet, and in the book of Gad the seer, [book: or, history: Heb. words]