The fact is that books are meant to be used, and in doing so accidents occassionally happen. Books that have been labeled "terminally damaged" by their owners have often earned me the name of "Consultant Book Surgeon" or "The Book Doctor" especially when the "patient" comes out of "intensive care".Mildew can also be a deciding factor whether the "patient" lives or dies.
There are many types of mildew and all are commonly known as "Foxing", the mildew gets its name from the (F)errous (Ox)ide or the iron it is attracted to in the paper, usually chemical wood pulp, and all are activated by the same cause...relative humidity.
This is a mild case affecting mainly around the edges.
Whereas the severety of this case is pretty much beyond belief!!!... but this "Book Doctor" has spent the past 30 years developing and implementing unique formulae for every kind of book restoration......................successfully.