Yeah, I was wondering where that $75 figure came from too. But it's even lower than $55 at the moment; when I checked out the product page there was special offer stating "Take 30% off any book with the code HOLIDAY30". So that brought it down to $42.21 plus tax. Not bad!
EDIT: Yes, $42 is more than 70% of $53. Not sure why the coupon didn't take off the full price. Actual price did come out to $42.21, though, for whatever reason.
EDIT 2: Ah, OK. Details for the coupon code says it maxes out at a $10 credit.