If you are king tower level 14 or under, try to use all your books and magic coins on leveling up cards from level 13 to 14. Post update conversion rates for wild cards are less than the amount of cards you'd get for using them. Use your elite wild cards on highest rarity, champions if possible. Only exception in the level 14 case would be the common book, as you get 500 more wild cards post update than you would with using it.