Microsoft, the advertising company, said it had understood the book was not available in the UK but had since realised this was not the case.
It confirmed the discount was limited to the US, which it said was explained when consumers visited the relevant website.
The ASA felt the ad did not go far enough to clarify that the discount applied only to exams taken in the US.
Therefore it felt the promotion was misleading.
The ad must not appear again in its current format.