Help me with my math? 5k lg gold.

Joe went to the book store and the clerk asked him to join the club to get 10% off. It costs $15 to join theclub. He thought about it for a minute, and then discovered that it would cost him $8 more to buy his books if he joined the club.

Question: How much did Joe's book cost without joining the club? Show how you found your answer(type it out)