From RWP Wiki
Bonus Books are major items in Grabbed by the Ghoulies.
Bonus Books are blue books with the Rare logo on them.
In the Games
Grabbed by the Ghoulies
In Grabbed by the Ghoulies, there are 100 Bonus Books total for the player to collect. One Bonus Book is found every time Cooper visits a room (for example, when he returns to a previously-visited room later in the story, there will be a new Bonus Book hiding in the room). Each five Bonus Books collected unlocks a new Bonus Challenge.