According to the author of the book of 2 Chronicles, this verse tells us that there are other stories about King Solomon written by different people like Nathan, Ahijah, and Iddo. It helps us know that Solomon's life was very interesting and many people wrote about the things he did.
2 Chronicles 9:29: Now the rest of the acts of Solomon, first and last, [are] they not written in the book of Nathan the prophet, and in the prophecy of Ahijah the Shilonite, and in the visions of Iddo the seer against Jeroboam the son of Nebat? [book: Heb. words]
This verse says that the accounts of Nathan, Gad, and Samuel are sources for the acts of King David, whereas 2 Chronicles 9:29 mentions Nathan, Ahijah, and Iddo for the acts of Solomon.
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]