How many book are in the Old Testament?
Depending on how you count it, there are either 24 or 39 books in the Old Testament. Adding in the apocrypha brings the number as high as 46.
The Hebrew Bible contains 24 books. English Bibles contain all those same books, but some of the books are split up giving a total of 39 books in English Protestant Bibles. For example, the book of Samuel is counted as one book in the Hebrew Bible, but 1 Samuel and 2 Samuel are counted separately in the English tradition.
Roman Catholic and Greek Orthodox Bibles also include several bonus books (“called the Apocrypha” or “Deuterocanonical books”) that are not found in the Hebrew Bible. Protestant churches like the Anglican Church respect these bonus books as containing helpful instruction (sometimes printing them in an appendix), but do not consider them as authoritative as the rest of the Bible.
Hebrew Bible
The Hebrew Bible includes 24 books
Torah (5): Genesis, Exodus, Leviticus, Numbers and Deuteronomy
Former Prophets (4): Joshua, Judges, Samuel and Kings
Latter Prophets (4): Isaiah, Jeremiah, Ezekiel plus The Twelve (including Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi)
Writings (11): Psalms, Job, Proverbs, Ruth, Song of Songs, Ecclesiastes, Lamentations, Esther, Daniel, Ezra–Nehemiah, Chronicles
Protestant Old Testament
The Protestant Old Testament includes 39 books – all the books of the Hebrew Bible but counted differently.
Pentateuch (5): Genesis, Exodus, Leviticus, Numbers and Deuteronomy
Histories (12): Joshua, Judges, Ruth, 1–2 Samuel, 1–2 Kings, 1–2 Chronicles, Ezra, Nehemiah and Esther
Poetic Books (5): Job, Psalms, Proverbs, Ecclesiastes and Song of Songs
Prophets (17): Isaiah, Jeremiah, Lamentations, Ezekiel, Daniel, Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi
Catholic Old Testament
The Catholic Old Testament generally includes 46 books: all the books of the Hebrew Bible plus 7 bonus books.
These bonus books are Judith, Tobit, 1 and 2 Maccabees, Sirach (also called Ecclesiasticus), Wisdom of Solomon, Baruch (including Letter of Jeremiah). Susanna and Bel and the Dragon are also normally included as additions to Daniel.
Eastern Orthodox Old Testament
The Eastern Orthodox Old Testament varies but generally includes 49 books: all the books of the Hebrew Bible plus 10 (or more) bonus books.
In addition to the bonus books of the Catholic Old Testament these bonus books include 1 Esdras, and 3 Maccabees. The Letter of Jeremiah appears on its own (not together with Baruch as in the Catholic Bible). Some versions add 4 Maccabees, Psalms of Solomon bringing the total to 51. Occasionally you’ll find Odes and Prayer of Manasseh. Susanna and Bel and the Dragon are often appended to Daniel. Slavonic Bibles are similar but also include 2 Esdras.
Ethiopian Orthodox Old Testament
The Ethiopian Orthodox Old Testament generally includes 46 books: all the books of the Hebrew Bible plus bonus books including 1 Enoch, Jubilees, 4 Baruch (included with Lamentations in Säqoqawä Eremyas), Ezra Apocalypse, and Ethiopian Maccabees. There is also a broader canon which includes Pseudo-Josephus.
The Letter of Jeremiah is often included in Baruch, and the Prayer of Manasseh is often appended to 2 Chronicles.
Summary
All the traditions recognise the same core Old Testament books of the Hebrew Bible, counted either as 24 or 39 books. Variation between Roman Catholic, Protestant and Orthodox traditions comes down to whether bonus books are considered as having the same authority, or seen as secondary books – useful, but not equally as authoritative.