The Monster Book was a gallery/information book of monsters you can encounter in the world of Vesteria, allowing you to see drops, health, and level. To unlock an entry, you simply have to collect a Monster Idol, which randomly drops from mobs.
Each monster gets extra XP per kill depending on the amount of Idols collected for that monster, stacking up to +50%, as well as more loot. The max amount of Idols for a monster depends on the tier, with Green being 5, Light Blue being 10, and Dark Blue being 15. Note that not collecting a 15th Idol and having a 16th drop, and collecting both, DOES NOT bypass the +50% XP limit. Some loot can only be obtained after collecting a certain amount of idols. Each unobtainable item appears with a lock, saying "Locked [x Idols]".
To see specific drops with drop chance and idol requirements, check the individual page of each mob (NOT FINISHED).