The Element Encyclopedia of Witchcraft: The Complete A-Z for the Entire Magical World - Judika Illes 2005
Books of Magic and Witchcraft
Very little is known today about the Druids and even less is completely understood; we’re not even 100 percent sure of exactly who they were or what their function was in Celtic society. (Yes, yes, I know. You can read a book that will definitively, absolutely identify the Druids, their beliefs, rituals, and actions; afterwards, you can pick up another book that will give you some other definitions and explanations. Many definitive versions exist.) Most of what we do know is filtered through the writings of Romans like Julius Caesar, who encountered them under less than friendly circumstances.
The standard explanation for our lack of knowledge regarding Druids is that they didn’t write, therefore left no information behind. Celtic society was either non-literate or it was felt that it was too dangerous to commit information, particularly esoteric, valuable information, to paper, where anyone could theoretically read it, including the uninitiated or enemies. This standard explanation may very well be correct; certainly no proof currently exists to indicate that it isn’t. The only clue that this might not entirely be the case comes in reports that St Patrick claimed to have burned 180 books belonging to Irish Druids.