The optional 5th book is the Deuterocanonical Books (commonly called the Apocrypha).
These are ancient Jewish texts that were written (mostly) in Greek. Therefore, they are not considered part of the Hebrew Bible (which makes up the Protestant Old Testament), and they mostly predate the New Testament. They are sometimes referred to as "intertestamental," but are more commonly called Deuterocanonical ("belonging to the second canon"). These texts are invaluable to our knowledge of this period of Jewish history and literature and in understanding the world in which the New Testament authors lived. And, like the rest, it's great reading. There are a wide variety of beliefs regarding this literature. Whatever your beliefs, Adam, the founder of Bibliotheca, agrees with Jerome: they deserve to be read.
Here's a list of the titles according to what's included in the English Revised Version:
Tobit, Judith, additions to the Book of Esther, Wisdom, Ecclesiasticus, Baruch, The Epistle of Jeremiah, additions to the Book of Daniel (The Song of the Three Holy Children, Susanna, Bel and the Dragon), First Maccabees, Second Maccabees, First Esdras, The Prayer of Manasseh, and Second Esdras.