A book bought second hand from an online source arrived smelling strongly of mildew. It was so bad I wondered how the book sorters could even have tolerated it. I did get a replacement, but decided to try to remove the smell. Sunshine, clay kitty litter, baking soda, ozone, and newspaper are the curatives I've most often heard of for smelly books. I tried newspaper.

Since it was a children's picture book, about thirty two pages, I could tuck a single sheet of newspaper between every page. After the first three weeks of newspaper soaking up the smell (I guess), the book was remarkably better. So every few weeks I changed out the newspaper for fresh. It has been four months, and I'm really satisfied. We'll see what happens over the long run, though. I think the prospects are good, because I don't think the book had any active mildew growth, but had just been stored in the vicinity of something mildewed.

Give this a try if you run into the problem.