I store all my books in a closed (but not airtight) wall mounted cupboard and I try to remove any dust which I see on the books and the shelves. Despite this, due to high humidity the infrequently-read books are afflicted with book fungus. The pages have spots and become yellow, while the dust cover has tiny white spore groups growing on it.
With respect to prevention, the answers to this question are not viable for me, as I do not have access to open space for spray treatment or the ability to remove and re-fix the cupboard.
With respect to treatment, I found many of the examples online to either be too mild, too severe or end up staining the pages, so tried and tested methods would be welcomed.