This verse is a formulaic closing statement commonly found in the books of Kings (and Chronicles) when summarizing the reign of a particular king. Let's break it down:

"Now the rest of the acts of Azariah, and all that he did..." This indicates that the preceding verses have only presented a summary of Azariah's (also known as Uzziah) reign. There was much more to his story than what's explicitly mentioned in 2 Kings.

"...aren't they written in the book of the chronicles of the kings of Judah?" This points to another historical record, "the book of the chronicles of the kings of Judah," as the source for a more complete account of Azariah's reign.

In essence, this verse means:

1. Incomplete Account: The information provided in 2 Kings about Azariah is not exhaustive.
2. Source Citation: A more detailed historical record existed and was used as a source for the author of Kings. This "book of the chronicles of the kings of Judah" contained more information about Azariah's activities.
3. Historical Context: It reinforces the idea that the books of Kings are selective accounts, emphasizing religious and moral aspects rather than providing a comprehensive historical record.

Important Considerations:

The Book of Chronicles: The "book of the chronicles of the kings of Judah" is likely not the same as the biblical book of Chronicles we have today. It was likely a royal archive or court record kept by the Judahite kings. The biblical books of Chronicles also drew upon these earlier historical sources.
Purpose of the Verse: The verse serves to authenticate the account in Kings by connecting it to other known historical records. It also acknowledges that there is more information available elsewhere for those interested in a deeper dive.

Therefore, this verse is a reference indicating that the details omitted can be found in "the book of the chronicles of the kings of Judah".