Hi,
It seems that if you have to chose the tome of dead to prevent the immortal magus (you released from his sarcophagus), you have to have the tome of dead in your inventory.
If you have it in a bag, you cannot go look for it because you can't use "bags" during this kind of moment (because of battle going on, i assume).
This is boring because the "put in bag" is not done to say "i don't want to use it" but because the game provides so few spaces for characters to hold things...