1 & 2 Samuel
Originally, the books we call 1 and 2 Samuel were just one book. But as early as the third century B.C., the book of Samuel was split into two parts by the men who produced the Septuagint—a Greek version of the Old Testament.
The books of 1 and 2 Samuel are a collection of sacred narratives. Who brought them together we can't say for sure. But they form an essential part of Scripture. First Samuel tells the story of Samuel's birth and his ministry to Israel as judge and prophet. It describes the transition the nation of Israel made from being a theocracy to becoming a monarchy under Saul, its first king.
Second Samuel, of course, continues the story and records the entire reign of King David, Saul's successor.
Some important theological themes in 1 and 2 Samuel have to do with the Holy Spirit, God's kingdom, and His matchless goodness.