Like Ragitsu said, we don't even need a "book" to understand the basics of Shar. A decent description of Shar that would be enough to cover the basic knowledge that most residents of Faêrun possess could easily be done in one sentence, or two (and so many people here have already made really good examples of such sentences). Personally I am in favor for having the narrator announce the basics whenever Shar is brought up for the first time - as the narrator is supposed to back the player with information that they should possess ingame, yet might not do so irl. Like a DM!
"Be inside when the night shows its face, Shar takes boys and girls that run into her embrace."
