Rarely is light completely responsible for a book’s destruction. If entropy had a gang and ordered a hit on a guy, light would feed the victim hamburgers until he died of organ failure. Thus the autopsy report would read “organ failure” and light could stand by looking all innocent.
There are three main symptoms of light damage: colour fading, yellowing and loss of strength. The displays in travel agent windows are a perfect example of the first — why do all those tropical paradises look strangely blue and unattractive? The red and yellow colours are more light-sensitive and fade first. (Change your pictures, travel agents!). A newspaper is a perfect example of the second two symptoms. Leave a newspaper out in the sun and in hours the paper will have acquired a yellow cast. Leave it longer and eventually you’ll be able to crumble the paper between your fingers.
The tricky thing is that not all books respond to light damage at the same rate.
Put two books in a window, side by side. The paperback will turn yellow quickly, eventually crumbling and chipping like the newspaper. The eighteenth-century book, printed on paper made from cotton rags, will probably look fine. The book bound in red cloth will fade along the spine but the book bound in leather may not show any change at all. Or, it might darken, rather than fade.
This doesn’t mean the books that look the same aren’t being changed — far from it. Light damage is cumulative, and each particle of energy hitting the surface of paper, cardboard, book cloth and leather is being absorbed. Molecules are still being altered. They are still being force-fed hamburgers and one day the damage might show. But some materials are less tolerant of force-feeding — organic dyes (more often the reds and yellows) and wood pulp paper have less sturdy internal organs.
However, other things change molecules, too — heat, moisture, rogue chemical agents! Light is just one member of entropy’s gang, remember? Entropy’s minions rarely work in isolation. They amplify each others’ effects. Working in tandem, their victim’s death is assured.
Miss Marple would not approve. She would call it clever, but wicked.