It took me too long to find the purchase link. I eventually clicked on the picture of the book and that worked. I was looking for another buy link for too long though (really, only 20 seconds maybe, but that's probably too long).
I always get annoyed when I navigate to a page and there are HUGE buttons telling me to spend money. So I tried to not do that.
Although, I may have pushed it too far in the opposite direction.