I've been a bookbinder for 35 years, mainly working with the secondhand book trade in the UK, making damaged/distressed books saleable.
I've tried many things over the years to get rid of bad smells that books pick up, and fresh out the bag cat litter works - don't use secondhand! It's inert and does no damage, which is the place to start, and it's cheap and readily available.
If the book has been at the bottom of a damp cellar, it probably won't make any difference, but that would definitely need professional attention.