That other book is, of course, John.  Now, I believe you know this is the Gospel of John, and the reason it is not included in the numbering system is because it is more than just a letter written to a certain individual or group to wish them well and address a certain issue.  This larger book is a telling of the life of Jesus Christ, and thus, it is put together with Matthew, Mark and Luke.

What Bible division is 3 John in?

-The Gospels-

-NT History-

-Pauline Epistles-

-General Epistles-

-NT Prophecy-