Illegal to take photos of covers within bookstores?

Well, since you're IN the bookstore, they can forbid you anything inside, they can even "force" you out the bookstore if they wanted to, since it's theirs.

So no, it's not illegal, but since the building is theirs, they can forbid you taking photos.

Also, taking photos in a shop isn't something owners like, since you could be planning a robbery and look for exit roots with photos back home, etc, etc.