Let There Not Be Light
Unlike peanut butter and jelly, light and books are not a good match. Light, whether it's natural or artificial, will trigger a chemical reaction in paper and paper-based binding materials that will ultimately break down cellulose. It's no friend to leather either. Colors will fade. Pages will become weak and brittle, and the resulting damage is irreversible. Nobody likes sunned backstrips or boards bearing the footprint of a neighboring book.
Some kinds of light are worse than others. Here is BookThink's Ladder of Light. The list descends from most destructive to least destructive.
BookThink's Ladder of Light
Direct sunlight. Direct sunlight is not only brighter than other forms of light but also contains a higher percentage of ultraviolet light, which is significantly more forceful and intrusive than visible light. Glass filters some of this out but not nearly enough to matter. There's only one thing to remember here: keep books out of direct sunlight.
Indirect sunlight. Ok, it's bounced around some, dispersed, and so on, but it's still destructive. UV light, in particular, is still afoot, and books in sun-drenched rooms, even if placed out of direct sunlight, will soon suffer.
Fluorescent light. Surprisingly destructive. Though not as damaging as sunlight, fluorescent light still contains a significant percentage of ultraviolet light, something on the order of 5 times as much as incandescent light.
Incandescent light. The best of the lot but still culpable. Several months of exposure to even relatively dim incandescent light is equivalent to a day or two on the beach.
Tips for reducing light damage: If possible, use incandescent light, and choose the lowest wattage practical. Hooded lamps that focus small pools of light on work areas are helpful in reducing the overall light content of a room. If fluorescent lighting is used, again, keep wattages as low as possible, and UV filters definitely help the cause. They can either be wrapped around the bulbs themselves or placed inside the covers of fixtures. Also, window film will help reduce the effects of sunlight to some extent, as will dark blinds or shades. If you're fortunate enough to own shelving with glass doors, use this for your most valuable books. It's another layer of protection.
Questions or comments?