Three official versions or rules collections exist for D&D: the Basic Set and its companion extension, the Expert Set; the Original or Collectors Edition; and the Advanced D&D series. This makes matters rather confusing to newcomers, as the various supplements and playing aids apply to different versions of the game. In practice, however, the Original edition is obsolete, and only of interest to veteran players and collectors. Players generally familiarize themselves with the Basic Set and then progress to the Expert Set (though the Expert Set is often by-passed), eventually moving to Advanced D&D, where the full scope of the game is realized.
I'm reading Ian Livingstone's 1982 introduction to roleplaying games, Dicing With Dragons, in my lunch breaks at work and the above passage jumped out at me for some reason. The Americanised spelling and missing apostrophe are Livingstone's doing.