All that are not the Four Gospels, Acts, and Revelation.
There are no books called "letters" (or epistles) in the Hebrew Old Testament. All books that are designated as "letters" are Greek, and found in the New Testament.
The 6 books of the New Testament that are not letters include: Matthew, Mark, Luke, John, Acts, and Revelation.
Historical, letters, and prophetic books.
21
Letters
It is the letters by Paul.
There are 21 letters written in the New Testament. Romans to Jude
Matthew, Mark, Luke, John, Acts, and Revelation are the books that are not letters/epistles in the New Testament.
There are books in the Old Testament, letters are in the New Testament.
The term used is 'epistles'.
If by 'works' you mean writings such as books or letters, then there are 27 in all.
That BC was a long time ago ;)