The division of nouns into "countable" vs. "not countable" is an artificial one. Many nouns can be placed in either category, even classic examples like "water" or "salt". That is why dictionaries do not state these categories in their definitions. If it were an important thing to know, then dictionaries would tell you to which category each noun belonged. That's not to say that the categories are not real. It just means that the category is determined by the way you think about the noun, and that that can change.

I am not an English teacher, but I am from the United States. If I heard "materials" or "material" in the example sentence, I would not correct either. Both sound right and would be used by native speakers. The difference between the "books" example and the "material" example may be that books are always countable whereas material is a bit more abstract.