Albert Barnes Commentary 2 Samuel 21:2

"And the king called the Gibeonites, and said unto them (now the Gibeonites were not of the children of Israel, but of the remnant of the Amorites; and the children of Israel had sworn unto them: and Saul sought to slay them in his zeal for the children of Israel and Judah);" — 2 Samuel 21:2 (ASV)

The way the writer here refers to the history of the league with the Gibeonites (Joshua 9) shows that the Book of Joshua was not part of the same work as the Books of Samuel.

Of the Amorites—The Gibeonites were Hivites (Joshua 9:7; Joshua 11:19), and in many enumerations of the Canaanite nations, the Hivites are distinguished from the Amorites. But “Amorite” is often used in a more comprehensive sense, equivalent to “Canaanite” (Deuteronomy 1:27), and denotes especially that part of the Canaanite nation which lived in the hill country (Numbers 13:29; Deuteronomy 1:7, 20, 24), and so includes the Hivites.