The general rule is to use an article for countable nouns (i.e. nouns permitting a plural) and no article for uncountable nouns. However, some nouns may be countable in one sense and uncountable in another sense, cf. the discussion of "truth" in http://www.oxforddictionaries.com/words/countable-nouns
Sometimes it seems hard for me to distinguish countable and uncountable senses, as the transitions between the meanings can be very subtle.
I recently stumbled upon the following:
This book is an essential reading. An extensive use is made of illustrations.
I found this incorrect and cut the articles:
This book is essential reading. Extensive use is made of illustrations.
However, the author objected and I was a bit troubled to explain, as both reading and use appear to appear in countable and uncountable senses.
Are both variants perhaps justifiable, or if the second is wrong, why exactly?
Or put differently, how can one distinguish the countable vs. uncountable meanings of "reading" and "use"?