What does Spell Codex do?

[Jaden (AceAngel TTV)]

Then are a few books that just shoot out a skull and nothing happens, what am I missing? Thanks.

[Rabbit]

The books spells randomize each run. Each run the different book names do something new.

[Ramond]

Some of the spells require you to hit an enemy with the skull to have an effect, a few from the tip of my mind:

-confuse the monster
-transform the monster into a random other monster (very useful in later levels)