The amount of ego it adds is its square root multiplied by 50.
The higher level, the more XP is needed to level up.
The books provide 4 to 2500 XP depending on their type and rarity, while the fights provide 4, 5 or 6 XP depending on the level of your opponent.
If you give it to her, she evolves and gains a star.
This is your main source of income.
You are ranked by it in the Tower of Fame and it also provides a bonus to your ego.