Art of War books let you upgrade particular hero class stats at the War Academy. You can get them by participating in Clan Wars. Each class upgrade requires a certain type of book.
You can only reset upgrades and get the resources back for free once. After that, you'll need to spend Gems.
War Academy upgrades let you boost the following hero stats:
- Attack
- Defense
- Health
- Damage to a certain class
- Resistance to damage from a certain class
- Attack and health
- Defense and health
- Attack and defense
NOTE: If you leave a clan or join a new one, you'll keep your books and upgrades.