I currently have a hero book in my clan castle. I am about to complete the golem challenge which will reward me a hero book. What will happen if I complete the challenge with no spot for the book?
If it is completing an event, the book will be added to your clan castle(Town Hall in the new update). 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.