yes be really careful with 'spots' in books - these are living, growing mold spores that will continue to grow and spread - you have to stop them.
i found this method online and have been using it for a long time.
i use the really big size zipper freezer bags, put in the book - no more than 3 or so to a bag, sprinkle in a little baking soda (not a lot, just a little) and seal tight. good advantage to adding baking soda is you can turn the bag and shake it and if none comes out the bag is sealed. FREEZE the books. we have a chest freezer that shows the temperature and you don't want to just cool the books, you want to freeze them.
after a couple weeks, i change out the bags and baking soda and freeze again.
the baking soda will absorb the odors. the freezing will kill most of the mold. i guess it won't kill mold spores in a lab test. but it's pretty effective for what i need.
when finished, just clear out the baking soda, wipe them down with a clean cloth (old tshirts are great for this)
i have a few really old books that i've rescued this way. they have value to me ... but probably not on the market..