That is pretty unbelieveable!
When the first edition of Bloch was written,
Java didn't have static imports yet, and inheriting an interface like this was therefore the only way to use the unadorned constants. Now you could say
import static ShowcaseConstants.CONSTANT_ONE;
import static ShowcaseConstants.CONSTANT_TWO;
import static ShowcaseConstants.CONSTANT_THREE;
and use only the ones you want, instead of "implementing" the interface. Personally, I think this is absolutely fine, and you might even consider it convenient to have all those constants in one place.
Block says you could put these in uninstantiable final classes instead of interfaces, but then you can't use multiple inheritance to compose them this way.