Before computers, librarians organized book information about the cards stored in card catalogs. Each book has several cards with details such as title, author, subject, etc., organized alphabetically for easy navigation by customers. Catalogers created human-generated metadata.
h/t:vintag.es
Librarians also used 'Librarian Only' shelf lists, which were cabinets with cards that were ordered like books on a shelf, marked with color-coded clips to indicate status (e.g. black = lost, blue = in repair). . While switching to the computer, the clip indicated which card was copied.
To mark special book locations without altering the cards, librarians used plastic covers with colored stripes with markings describing the color (e.g., red = reference shelf, green = children's books).