Well, his first appearance was in Detective Comics #66, which tells his origin. In that, the character's name was originally Harvey Kent (later changed to Harvey Dent because of associations with Clark Kent).
One of my favorite origin stories is "The Eye of the Beholder," which was a major influence for THE LONG HALLOWEEN and, in some senses, is a little better. It's more Dent-central, and goes into his relationship with his father more. I especially love the haunting end of this take on his origin - his face is restored, things are looking up, and then, with his bare hands, he scars himself again as insanity reclaims it's hold on him.
Both "The Eye of the Beholder" and Detective Comics #66 are collected in the volume BATMAN: FEATURING TWO-FACE AND THE RIDDLER.