Grabbed by the Ghoulies Cheats

Grabbed by the Ghoulies Unlockable: All Challenges
Collect 100 Rare Bonus Books to unlock all challenges.

Grabbed by the Ghoulies Unlockable: Bonus Challenge
Complete all challenges with a gold ranking or higher to unlock the 21st challenge.

Grabbed by the Ghoulies Unlockable: E3 Demo Video
Complete the 21st bonus challenge.