This class is a place to gather the methods that have to do with making thumbnails of pages of books. Three of the methods were previously methods of Book itself, but the fourth (MakeThumbnailOfCover) needed to do too much UI stuff to belong in a model class, so it seemed worth pulling all this out to a new class. In live code there is typically only one instance of this created by the ApplicationServer. In test code they may be created as needed; nothing requires this to be a singleton. Indeed, it could be a static class except that it requires the HtmlThumbNailer.