Another hand raised from one who used to work in libraries.

Joe, the musty smell can be from the glue used, or some types of papers just seem to absorb more of the air around them.....particularly in a humid environment.

Slightly off-topic, but related: one of my sewing machines is a 1950 Singer Featherweight. Featherweight owners have tried many things over the years to rid the carrying/storage cases of a musty odour, and most seem to have decided that it can be lessened but never completely removed. Perhaps it's a combination of the case being closed when the machine is not in use so it doesn't get aired, old oil which has turned slightly rancid, the glue used to stick the imitation leather lining and outer to the case - any or all of the above. Leaving the case opened outside on a sunny was said to help, but once it was closed again the smell returned.

Unused kitty litter is said to be good for absorbing odours and moisture, but I haven't tried it.