Leaving the point of whether it's unfair to include lore from books aside for the moment, there is one book in the game that alludes to this but I can't remember where I found it. It's the story of young woman who hears Shars voice and follows her instructions to throw a vial down a well and in doing so earns Shar's love. The moral of the story is that people come to see her not because she is attractive but because she is high priest of Shar.
Now I am with you @abits in saying this could be better supported. I would like the books to be longer and for there to be more of them. You could learn everything you needed to know about FR lore in BG1 by raiding the shelves of Candlekeep and Durlag's tower.
But returning to the outside lore issue D&D has always been this way -- there is salty bit at the end of the 1st edition DM guide where it tells you can't understand D&D unless you read fantasy books and then gives you a reading list.
