What does it look like ? I got a book today and the top of the book on the outside all the pages are like little dots.............is this mildew ?

Could be some kind of mold. I got a book like this once. It had clearly been wet at some point. All the areas that had water damage had these mold spots all over them.

If they are yellowish to brown, it is probably more likely to be foxing (natural aging process of the paper). This is postable.

If it is grayish or whitish, I would think probably mold or mildew. Does it smell like mold? If the book has visible spots on it, that are NOT foxing, then it is probably unpostable.