You'll need radiant damage to destroy the book, with shadowheart's guiding bolt being the most reliable option early game. Watch for the book's dark aura to pulse which signals it's vulnerable, then time your cast right as the pulse peaks for maximum effect. Position yourself away from nearby containers since the destruction triggers three shadow enemies that'll immediately aggro - they're invisible and deadly at low levels so having clear space to dodge is crucial. The shadows typically spread out in a triangle formation, so backing up against a wall limits their flanking angles.

When you're ready to destroy it, make sure your party is buffed and positioned since the fight starts instantly after the book burns. The shadows have a specific attack pattern where they drift back slightly before lunging, so time your defensive moves when you see that tell. Keeping Light cantrips ready helps reveal their positions, and area of effect spells can catch their movement patterns even while invisible.

Maybe interesting - failing the destruction actually curses you with disadvantage on wisdom saves, but succeeding gains approval from most companions except astarion who gets pretty upset. You do permanently miss out on some strong abilities like free speak with dead and the tharchiate codex's temp HP bonus later, but if you're going for a good playthrough the trade-off makes sense story-wise.