Library of Congress Classification System
Classifying a Personal or Small Library
If you're going to classify books in your personal or small library, you'll want to use an established classification system. I recommend the Library of Congress Classification System due to the ease of classifying.
Basically, utilize an on-line catalog from a library, search for the title, and copy the call number onto a sticker or other label for the book. I hand write call numbers on a piece of index card and tape the label to the spine of the book. The Library of Congress' LOCIS system is a public access catalog which you can telnet into (telnet locis.loc.gov). See their Catalog page for more information.
This simple method will allow you to organize your small library just like the big ones. Good luck!