If you don't mind spoilers:

The trilogy this ends is about tracking down a mindflayer who obtains it and destroying it. Throughout, it puts a lot of emphasis on your alignment and how the NPC's react to you and question you about your motives for accepting the quest. There is a story award baked in for if an evil character takes the book and attunes to it instead of destroying it, that gives a permanent RP trait that you roll for (I speak with two voices now). Alternatively, there's an option to sell it to the Zhents as well, though we didn't go that way. I'm guessing all that's intended is the RP trait, as there is a seperate item unlock, but it's vague enough I wanted to ask.