“The other events of Jeroboam’s reign, his wars and how he ruled, are written in the book of the annals of the kings of Israel.” (1 Kings 14:19)
“As for the other events of Rehoboam’s reign, and all he did, are they not written in the book of the annals of the kings of Judah?” (1 Kings 14:29)
Because the kingdom was once divided for many years, two books recorded the acts of the reigning kings from the different regions, one being Judah and the other Israel. Examples of these two books can be found extensively throughout both Kings and Chronicles. For example in I Kings 14:19 (Book of the Annals of the Kings of Israel) and in the same chapter but verse 29 (Book of the Annals of the Kings of Judah).
Just what are these Annals?
These historical records of both Judah and Israel were the primary sources of material God directed the author(s) to use to write 1 and 2 Kings. These state annals were used for every king to at least the end of the reign of Jehoiakim. It is presumed that these books contained records of the reigning kings, register of achievements, and important calendar of events. It is also quite possible that they contained records of peace accords and other transactions of royal governance. No copies of these books have ever been found.
In conclusion, these mysterious books were, for whatever purpose, divinely selected to not be a part of the official canon of Scripture. They can be considered as simply “extra-biblical references”. What makes them so fascinating however is that they are in fact mentioned in the Bible from the authors themselves, i.e. Jude, Joshua, Moses, Jeremiah. Because the titles of the books themselves are mentioned throughout the Bible, they are considered to be a small part of the inspired Word of God.