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.
Well after playing Skyrim, dragon age, and the Witcher games mostly without reading any lore bit and still have a very good idea of what's going on in the world, I would say it's a rather weak excuse. The same is true for the lord of the rings for example. You can read the lord of the rings without delving into the appendices or the silmarillion and not feel like you missed anything.
Now the Forgotten Realms lore is vast, really vast. But it's the game's writers job to figure out what is important to the story they tell and what isn't. And then they should figure out the best way to explain this bits to the player.
