Your own Library of Congress.

I want my nonfiction books organized according to some widely-accepted classification system like the Dewey Decimal System or the Library of Congress Classification, but defacing my precious books with tasteless spine labels is a non-starter.

These days, most books come with a label already printed on the back: the International Standard Book Number, or ISBN. Wikiepedia gives the confidence-inspiring assurance that these numbers are "intended to be unique", so we should be able to build a system that matches a book's ISBN to its classification and tells us where to shelve it.

This project was originally called Dewey, after the famous librarian and classification system. A quick skim of his Wikipedia page reveals that he's not the kind of guy one should name things after. So instead I'm calling it Putty, after Herbert Putnam, the longest-serving Librarian of Congress who is remembered for his "impenetrable dignity" and "covered shyness".