Put the facts together, guys:
1) Leveling normally takes praying to a god once you've got enough XP.
2) Your god will decide what you get from the level up.
3) The world book originally belonged to the goddess.
From this we can infer that gods write in their respective books (I assume there's more than one) in order to grand the level ups. So he'd just be using that function, it doesn't necessarily mean he has the power to change anything else.