What is the oldest book in the New Testament?

The first book in the New Testament (and, at least according to the Catholic Encyclopedia, the first GOSPEL written Chronologically, is the Gospel of Matthew).

The same source suggests that the Epistle from James may be the first book to have been written, though it also suggests that this may still be under dispute.

Source: Catholic Encyclopedia at

http://www.newadvent.org/cathen/14530a.htm

An interesting read: For about sixty years now a tiny papyrus fragment of the Gospel of John has been the oldest "manuscript" of the New Testament. This manuscript (P52) has generally been dated to ca. A.D. 125. This fact alone proved that the original Gospel of John was written earlier, viz. in the first century A.D., as had always been upheld by conservative scholars. More at:

http://scriptorium.lib.duke.edu/papyrus/texts/manuscripts.html

Of all the surviving letters written by the Apostle Paul, 1 Thessalonians is very likely the first to have been written. This also makes 1 Thessalonians the oldest book in the New Testament. It was written in the early 50s, probably in 51 C.E.

http://www.enterthebible.org/Bible.aspx?rid=1

This source says probably James in A.D. 45.

http://bibleresources.org/bibleresources/bible-facts/