The spell book is a useful item to have, allowing mages to organize their spells and save space at the same time. The spell book, when equipped, can be scrolled through either by pressing and rolling the mouse wheel. Spell books can be dyed a variety of different colors. They can also be enchanted with Soulbound. (Not quite sure what else)
When you have the Spell book in your hand, press Shift and Right click to
open the spell book interface. By default, the Z and X keys cycle throught the available spells in the leftmost column. Another way of cycling through spells is Shift+scroll
There is a fairly common glitch where you will be unable to scroll through your spells, especially with Z and X, as it automatically crashes your instance of Minecraft. Be wary. This is also an issue with the Arcane Spell Book, an improved version of the spell book that rarely drops from the Arcane Guardian And you can dye it.