Reading it will grant you the Scroll of Artistry of War which can be learned by your Wizard. The Annals of Karsus Legendary Book Dethrone –(Level 5 Necromancy) 30-80 Necrotic Damage or half that when the enemy rolls a save. How to get The Annals of Karsus To get the Annals of Karsus...
The most obvious two are Gale and Wyll, who both mention or make use of spells far higher than they have access to in BG3. Gale, for example, uses the sixth-level wizard spell contingency should he be killed, with a mirror image version of him appearing and talking the party through ...