How about, the final installment sold 11 million copies in its first day. Or, this series was the first children's books on the NY Times bestseller's list since E.B White's Charlotte's Web (1952). And over 16 thousand kids tried out for the role of Harry Potter.
This infographic should give you some idea.
Update: At least one of the stats on this infographic is wrong, so I'd be careful about trusting the rest.