I must store my personal library in boxes for several months. What can I put in or around the boxes to keep out silver fish and spiders until I can unpack them?

First off, do you have to use cardboard, or do you use plastic? I have a storage container that I store books and other paper items in and I only use plastic containers due to previous issues with bugs/mice getting into everything.

Aside from that, have you tried cedar products? There are many out there like the cedar balls for closets, cedar oil dispensers, cedar chips, etc. Cedar would be my first choice to try for storage issues.

Also, if you don't mind getting the place a bit dusty, you might try dusting around the boxes and shelving with some DE. We use it in all the cracks and crevaces of our house to keep ants and other invertebrates from invading. Placing it around the boxes and on the shelf surfaces should help prevent them as well.

I agree with the plastic bin idea. I would dust the bottom of the bin with boric acid first. That seems to kill all insects.