We do not use an article in the sentence “books are expensive” because we are talking about books in general the noun ‘book’ is uncountable the noun ‘book’ is abstract we don’t want

we are talking about books in general

the noun ‘book’ is countable so we may use a/an or the with it. Whereas the noun ‘progress’ is abstract noun and the noun 'water' is uncountable so we use a zero article with them when talking about them in general.

Объяснение: