According to the author of the book of 2 Kings, this verse tells us that if we want to know more about King Jehoahaz and the things he did, we can find it written in another book called the chronicles of the kings of Israel. It's like when a story doesn't fit in one book, so more details are kept in another book.
2 Kings 13:8: Now the rest of the acts of Jehoahaz, and all that he did, and his might, [are] they not written in the book of the chronicles of the kings of Israel?
This verse describes the official records of Jeroboam's reign, while 2 Kings 13:8 refers to another set of acts, indicating differing accounts of historical events.
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.