The book by Melvil Dewey which serves as the basis for the Dewey Decimal System. It's available in an abridged format for your personal library or for small libraries, but medium and large libraries usually use the full Dewey.

The Dewey Decimal Classification and Relative Index is divided, as the name suggests, into two parts. The first part, the Dewey Decimal Classification, is ordered by the numbers (for example, E2 might be under 032, "General encyclopedic works in English"). The second part, the Relative Index, is in alphabetical order so that you can look up anthropology and see that it's somewhere around 573.

If this fascinates you, perhaps you have a not-very-lucrative career ahead of you in the field of librarianship.