Clash of clans hero book storage

Your second will be useless as it is clearly observed that Treasury have only 1 storage place for the Hero Book. So before completing challenge reward, make sure you have empty room for Hero Book.

If it is completing an event, the book will be added to your clan castle. Rewards for events stack so that you can have more than the maximum number! I currently have 9 power potions; the maximum is 5; 4 were rewards for completing challenges during events

If it is part of a clan game, it will not stack and you will not be able to choose it as a reward.

You can always have those rewards from events as the players have earned it by completing event so supercell does not take it away from us. I had 6 training potions and two book of fighting so no need to worry.