Usually refers to the brownish colour of fore- edges and pages of hardbacks or paperbacks, age tan which is caused either by fading or acid present in the paper.
CJ grew up admiring books. His family owned a small bookstore throughout his early childhood, and he would spend weekends flipping through book after book, always sure to read the ones that looked the most interesting. Not much has changed since then, except now some of those interesting books he picks off the shelf were designed by his company!