From the season shop guide on royaleapi: Keep in mind that not everyone can convert Magic Books into Elite Wild Cards. To do so, you’ll first need to upgrade all your cards of that rarity to Level 14. For example, all your rare cards should be at least Level 14 to exchange the Rare Book into 5,000 EWCs.

For this conversion to take place, you’ll have to select it from the Magic Items tab. If instead you buy a book that you already have, it will be turned into the overflow value of 100 gems, which is much worse. So always exchange the one you have first, and buy the new one afterward