The Supplementary Catalog contains scans of catalog cards created before 1980, with their original Richardson numbers. Records are arranged alphabetically with authors, titles, and subjects interfiled. This catalog has been largely replaced by the online Main Catalog.
Richardson call numbers in Firestone (those that begin with four-digit numerals) have been converted to Library of Congress call numbers (which are alpha-numeric).
If the need arises to search a title with a Richardson call number, it is possible to search the Main Catalog for the title (as a "title") or the Richardson call number itself (as a "keyword") in order to obtain the current location and call number information.
A thorough subject search of your topic is a great start to finding books in Princeton's online catalog. Some LC Subject headings have been provided here, but a complete listing of LC Subject Headings may also be found in pdfs online or in the catalog . You can also look up a known book on your topic and check the long view for the subject headings assigned to that book.