The only real plus side to this is that you don't need to get the Dark Amethyst to do it. These Shadows are highly resistant to physical attacks and fighting them is just generally a bad time.
Doing so will cause three very tough Shadow enemies to spawn and without a strong party, you'll die pretty quick. Considering this a tome of Necromancy, it presumably tells him something important, but there are no immediate consequences.
He'll approve of this and look through the book briefly before holding onto it. Give it to Astarion: Once opened, you can give the book to Astarion.You'll also be able to finish reading the book later in the game for some pretty wild bonuses. If you succeed at all three you'll get the Forbidden Knowledge trait, which gives you a permanent +1 to all Wisdom saving throws and ability checks, plus the Speak with Dead ability. Read it yourself: To learn the Necromancy of Thay's secrets, you'll have to pass three Wisdom skill checks of escalating difficulty-10, 15, and 20-as you turn each page in the book.
