most paper contains some amount of lignin, which is a polymer that naturally occurs in wood and acts as a stiffener. as lignin oxidizes, which is to say combines chemically with the oxygen in the air, it turns brown just like an apple. sunlight catalyzes this reaction, so a book left next to a window for three years will end up browner than a book left in a box for three years.

Oooohhh. OK. Thank you. :D