I learned about difference tables in Conway and Guy's great book "The Book of Numbers". I wish the book had been available when I was an undergrad. It describes everything I loved in math that I wasn't able to pinpoint. There are so many little gems in it, I've spent months reading it halfway through and then starting over to pick up what I missed. Highly recommended. http://www.amazon.com/Book-Numbers-John-H-Conway/dp/03879799...