Several hints here:
eBay Booksellers 

This is a new one to me and it's seems to work, too -

"We've tried many tricks to get the odor out. Kitty litter/baking soda/fresh air have had limited success.

 

We acidentally found what SEEMS to be a better method. We tucked the book into one of those cheap (99 cent store kind of cheap) plastic gallon bags (twist tie) in order to keep the smell from going into other books. Came back a couple months later and found that the bag itself had absorbed most of the odor. So we kept doing that with clean bags a couple more times and it worked fairly well. Not complete, but fairly well."

I would strongly suggest not to mix any musty paper with non-musty paper. The plastic may mitigate mold or mildew spores spread.