The Books of Samuel are part of the Tanakh (part of Judaism's Hebrew Bible) and also of the Christian Old Testament. The work was originally written in Hebrew, and the Book(s) of Samuel originally formed a single text, as they are often considered today in Hebrew bibles. Together with what is now referred to as the Book(s) of Kings, the translators who created the Greek Septuagint divided the text into four books, which they named the Books of the Kingdoms. In the Latin Vulgate version, these then became the Books of the Kings, thus 1 and 2 Samuel were referred to as 1 and 2 Kings, with 3 and 4 Kings being what are called 1 and 2 Kings by the King James Bible and its successors.
