In the past, and still today in some locations, libraries had halls or open rooms dedicated to their card catalogs. The large libraries had rows and rows of handsome wood cabinets, the drawers filled with index cards for each book in their system. The smaller libraries, such as that in a high school, might be able to catalog their books into one such cabinet.
There is something visually appealing about the cabinets with the rows and columns of drawers. The experience of using the catalogs is appealing as well. You may look up a book by the author's last name, or by the subject category, or by the book's title. Say that you are interested in finding 'Huckleberry Finn' by Mark Twain. The drawer label might read 'Tr through Un' for information on books by Mark Twain. Or you might look up 'Ma through Mo' looking for books regarding the subject 'Mississippi River'. If the library has that book, there will be a card for it with a code to help you know which part of the library, and on what shelf it is located. The code is called 'The Dewey Decimal System' and categorizes books according to general book type (Fiction, Juvenile, Reference, et cetera) and by author or other relevant information. The code label is usually attached to the spine of every library book.
With the advent of computers into libraries, we can quickly locate a book using the library's catalog computer. The computer has a word search tool into a data base of the library's inventory of books. The library keeps the data up-to-date. You can even discover if the book is present or has been checked out.
Some libraries have such a computer system, and, for many reasons, also keep their card catalogs active.
Cards referring to Mark Twain might be curled and softened by use across many years. More specialized or unfamiliar books might have cards still in pristine condition. The feel and scent of the old catalogs have a certain romance to it, for you are in touch with thousands of readers before you who have thumbed through the little cards in search of a book.