When I was at Borders, they had a master copy of this coupon in question which they showed me after I said it didn't specify just being for the 3 certain books. The manager explained saying it wasn't my fault for thinking it was good on any purchase, he showed me it was defined at the bottom of the page outside of the coupon itself. He said it was Borders fault for how it was put together and had no problem honoring it "just once" as he put it.
Borders is anything but cheap, the only time I ever spend money there is when I have a coupon.