OK first thing to say is if you have found a book that is coved in Mould then remove it from the others as this will contaminate and spread. Humidity is the number one condition for the growth of mould and mildew. It is the moisture in still, quiet air that allows mould spores to grow and spread. Think of dank basements, musty attics, or clothes left in the washer too long – these are prime mildew-growing habitats. But in a Bright Airy Library, there shouldn't be too much of a problem with mould or Mildew. A dehumidifier should help to keep the humidity under 60 percent, but only when necessary. Books that are too dry can be damaged and crack. Generally in a well ventilated Public Library there shouldn't be an issue with Mould, Unless its Dark, Dank, no air movement and high humidity. Whilst he has a concern, the chances of him being overcome by mould in a modern library in this day and age is very remote