I have no idea as well due to me just reaching that point into the game myself but I kinda want to open it to see what happens. While I was testing stuff and I discovered that (which is funny) if you grabbed the gems from the cellar room (the Laculite, Lolth's Candle and Purple Fluorite Shard) you can destroy the tome by selecting those gems (or any gems you have on hand) and just chuck at the tome. It will take damage from getting hit with the gem and finish the quest via destroying the tome. You are welcome for the funny solution xD
