What the Bible says about Ketuvim
(From Forerunner Commentary)
Jesus, using a common Jewish method of organizing the Scriptures (only the Old Testament at that time), breaks it down into three sections. Even today, Jews group them in these same sections: Law (Torah), Prophets (Nevi'im), and Psalms or Writings (Ketuvim). Thus, Hebrew Bibles are called Tanakh, a word made up by combining the three initial Hebrew letters of each major section of Scripture.
The section of the Prophets is itself divided into two major parts, the Former Prophets and the Latter Prophets. The Former Prophets are the historical books of Joshua, Judges, I & II Samuel, and I & II Kings. The Latter Prophets are the named prophets from Isaiah to Malachi (excluding Daniel, whose book is included in the Writings). The twelve works that comprise the Minor Prophets are a subset of these, which the Jews consider to be, not twelve little books, but one large book, the fourth of the Latter Prophets, balancing the four books of the Former Prophets (I & II Samuel are considered one book, as are I & II Kings). As such, the Minor Prophets were often written on one scroll.
Richard T. Ritenbaugh
Meet the Minor Prophets (Part One)