Books are consumable Items in Hordes.
There are 5 types of Books in Hordes, four can be used by different Classes to unlock Skills and one specifically to unlock Mount Riding for all Classes.
The Warrior Book can be recognized by its crimson color and can unlock Warrior Skills.
The Mage Book can be recognized by its light blue color and can unlock Mage Skills.
The Archer Book can be recognized by its light green color and can unlock Archer Skills.
The Shaman Book can be recognized by its dark blue color and can unlock Shaman Skills.
The Mount Riding Book can be recognized by its purple color and can unlock the Mount Riding Skill.