The Great Book of the Unknowing is a side quest which is started by talking to Shaella in the Blade and Stars. In order to receive the quest, you must always answer her with questions or statements that you don't understand what she wants. The correct dialogue options are 1,1,1,1,2,1.

To complete the quest, simply talk to her again and choose option 2. This will move the quest to the done quests section, although there is no reward for it.

All of this is only possible in Baldur's Gate: EE as in Baldur's Gate the quest doesn't even appear in your Journal.