Wizards of the Coast's Dungeon Master's Guide II, which is shipping this month, was ordered in the largest quantities of any Dungeons & Dragons book (other than the three core titles) since 2002, according to a Wizards of the Coast spokesperson.   This is the first time there's been a sequel to a core book in any edition of D&D (other than the Monster Manuals), but the sales seem to indicate that the trade expects that gamers will add DMG Guide II to their D&D libraries in near-core-book quantities. 

Dungeon Masters Guide II was written by Jesse Decker, David Noonan, Chris Thomasson, James Jacobs, and Robin D. Laws.