Why do some consider the minor prophets one book?

Answer:

The books that you refer to are commonly called the minor prophets. They are not minor because they are unimportant, but because their writings were short -- so short that back in the days when the Bible was written on scrolls, the minor prophets were kept on one scroll, known as the Book of the Twelve in the Hebrew Bible.

The influence of scrolls is shown in another way: I Samuel, II Samuel, I Kings and II Kings are actually all one work, but because of its size it was divided between four scrolls. The Hebrew Bible calls them 1, 2, 3, and 4 Kings. In our Bibles, 1 Kings is called I Samuel, 2 Kings is called II Samuel, 3 Kings is called I Kings, and 4 Kings is called II Kings. These four books, combined with Joshua and Judges are known collectively as "The Former Prophets." The books of Isaiah, Jeremiah, Ezekiel, and the Twelve were known collectively as "The Latter Prophets." Taken all together, the two sets make up the division known as "The Prophets" in the Hebrew Bible.