The XP Tomes are required to level up your heroes. There are 5 types of XP Tomes: Common, Uncommon, Rare, Epic, Legendary.

Collecting XP Tomes, you will be able to upgrade your heroes faster. If you do not have enough XP Tomes, don't worry! You can get  them for completing Daily Quests "Win 3 hard missions in the Order / Clans campaign", "Increase the level of any hero", "Buy 3 items in the shop" as well as various game activities, such as Campaigns,  “Combat Endurance” Challenge , Tower and Quests.

Each type of XP Tomes gives a different amount of experience points. For example, a Common XP Tome adds 40 experience points, Uncommon adds 200 points, Rare - 1000, Epic - 2000, and legendary - 4000 points.

If you don't have enough books for hero upgrade, you will see such a pop up that shows you the amount you have left. You can upgrade your heroes further by using Dracoins or by obtaining more XP Tomes.