Books that smell of mildew? I've had success with putting them in an air-tight, sealable, plastic food container (Tupperware?) and covering with cat litter. Check after a week and repeat if necessary. Remember to brush all the cat litter from the book before opening it, otherwise it could get between the pages.
One can do a repair on "perfect bound"* books but it's very time consuming and, consequently, expensive. See here for an example of what's required to make them last:
Unfortunately, we did't learn the lessons of those Victorian experiments with glued spines and most older paperbacks will suffer the same fate eventually. Although modern, synthetic glues are better than gutta-percha, who knows how long they'll last.
And, here in the UK, most modern hardback books are just paperbacks with stiffer boards. Proper, folded and sewn, hardbacks seem to only come out of the University Presses these days.
Humanity may regret an over-reliance on digital.
*Whoever came up with that name surely had a sense of humour!