Over 300 years ago, Don traveled to the Dark Continent. There he wrote a traveler's journal consisting of two volumes: "East" and "West." While the "East" edition is in the hands of V6, the "West" edition has yet to be found. According to Ging, it's because Don is still alive and is working on writing it.
Ging said it was one of 3 possibilities that he could think of. It is not confirmed if Don is still alive.