There are many Books in the Realm of DSZC. They can be found, looted, dropped or can even be gifts. The ones listed here actually have meaning and value and you should keep on the lookout for them. Some of these are typically only used in the games related to DSZC.
Book of Time: Books of Time are extremely rare, and are used to summon a single object from the past to the present. If amplified by a God's power, it can change an entire area, not just a single object. So an area that used to be a forest and is now a desert would be reverted to a forest. It would, however, only effect a small area.
Book of Space: Books of Space are just as rare as Books of Time. They're used to create pocket Realms for weak beings.
Book of Understanding: Books of Understanding are pretty common. They give the user experience. The amount depends on the game you're playing. If it's the DSCZ game, then it gives experience proportional to your level, if it's Minecraft, you simply get a couple EXP Bottles.
Book of Wealth: Books of Wealth are slightly more rare than Books of Understanding. They give the user money. The amount depends on the game like earlier. In the DSZC game, it gives money proportional to your combat level and in Minecraft, you get money based on your skill levels in total.
Book of Chaos: Books of Chaos are quite rare. They give the user a choice between a small box and a large chest. Each one leads to a different, random prize. Remember: Bigger doesn't always mean better!
Book of Stars: Books of Stars grant magical wishes. They can increase your overall power, or help you learn a new magic of some kind.
Book of Gates: Books of Gates allow the user to be transported to any location in existence. Whether they die when they get there is up to them, so making a good choice is key!