The Jews count the 12 minor prophets as one book, so you lose 11 books there, but not really.
They also count I Samuel and II Samuel as one book, so you lose another book there, but not really.
They also count I Kings and II Kings as one book, so you lose another book there, but not really.
They also count I Chronicles and II Chronicles as one book, so you lose another book there, but not really.
They also count Nehemiah and Ezra as one book, so you lose another book there, but not really.
Add those all together and there are your 15 "missing books", but not really
The 24 Books of the Hebrew Bible - Torah.org