Why do books turn yellow? It's all thanks to this complex molecule called "lignin." It's a dark substance found in wood, and which is responsible for the hardness of the wood. But when the wood is turned into paper, the lignin is retained, and it's the exposure of lignin to air and sunlight that causes the yellowing you see on your books.
Low-quality paper contains 20% lignin and they yellow easily.
Look only to your standard, daily newspaper for an example. High-quality papers, which are processed more thoroughly, contain only 1% of this substance, and they resist yellowing for a good long time.
But not all books are available in high-quality paper, so here's how you can keep them in tip-top condition:
Store them away from direct sunlight.
Ultraviolet rays cause fading on the covers and spines and promote yellowing of the pages much faster.
Store under moderate humidity.
Paper becomes brittle in a place that's too dry. On the other hand, paper can grow mold in an area with a lot of moisture in the air. Ideal humidity should be 50-60%.Use a humidifier or a dehumidifier in the room.
Allow for proper air circulation.
Leave a few inches of space between your books and the wall or the insides of your shelves.
This should keep your books well-ventilated and less prone to yellowing and developing mold.
Use archival paper between the pages of the book.
It's impractical for an entire library, but doable for a small collection or several of your most treasured or most expensive volumes. This special paper absorbs damaging acid from the pages, keeping them in top condition.
Some general tips and common knowledge go a long way: handle your books with clean hands; don't place them on dirty or wet surfaces; use plastic covers for added protection; dust and clean your library regularly; and make sure your books won't get bent or wrinkled if you're taking them with you in your bag.