In many books, on the page after the title page but before the "first" page, there is a page with copyright, publishing, and Library of Congress information (well, in American published books, at least). Towards the bottom, there is often a series of numbers like so: "1 3 5 7 9 8 6 4 2 0" or "1 2 3 4 5" or "7 9 0 8 6". In a book I recently purchased, it is "A B C D E F".
What are these sequences? What do they mean? What is their purpose?
I am in the planning process of a story involving libraries, secret societies, codes, and a Nancy Drew type figure, and right now I'm trying to work out different ways that the secret society could communicate, well, secretly, but still leave traces for the girl to find. So these sequences might be useful for that.