You would think that books in a fallout shelters would be to help prevent boredom during a long nuclear winter, but no.
The U.S. Federal Emergency Management Agency didn’t actually suggest that books be used, but merely provided a general guideline for substitutions if concrete and other standard building materials weren’t available.
Here is an excerpt from the FEMA handbook In Time of Emergency:
Concrete, bricks, earth and sand are some of the materials that are dense or heavy enough to provide fallout protection.
For comparative purposes, 4 inches of concrete would provide the same shielding density as:
5 to 6 inches of bricks
6 inches of sand or gravel or 7 inches of earth (may be packed into bags, boxes, cartons for easier handling)
8 inches of hollow concrete blocks (6 inches if filled with sand)
10 inches of water
14 inches of books or magazines
18 inches of wood